I've tested that phpvirtualbox does properly write and remove this setting from the VM config. I am unable to test if this feature actually works because it is currently only supported on AMD CPUs, which I don't have. See https://forums.virtualbox.org/viewtopic.php?f=1&t=90831 for more info about limitations with Nested Virtualization on VirtualBox. The actual commits look proper to me and I guess this can be merged.

Tested on AMD Phenom 9650. This function working

@ZIM555 thank you for testing. @h6w anything else needs done before merge?

Nested VM are supported also for Intel in VBox 6.1

Sorry for the delay. Excellent work @chotaire . Thanks for bringing the good stuff in from @pasha1st's work. :-)

Oh, before I accept it, can you have a look at why the entire of vboxconnector.php is now in conflict?

a) I merged current vboxconnector.php https://github.com/phpvirtualbox/phpvirtualbox/blob/develop/endpoints/lib/vboxconnector.php from develop branch into my PR.
b) I merged the previously proposed nested Virtualization patches again.
c) Fixed typo reported by trasherdk (in now superceded PR #217)
d) Would still give a merge conflict on the entire file. I diffed it externally, saw nothing wrong, chose my new version, resolved conflict. I have actually no idea what suddenly made Github report a conflict. My only guesses are either line endings ignored by my diff tools or a github error.
e) Diffed vboxconnector.php from bc88fca again with current vboxconnector.php from develop branch, looks like proper changes to me.

@h6w: If preferred, I can delete and create a new PR, should only take a few minutes.
PS. And no, I use VS Code, not this editor. I just quickly used it for diffing.

I just checked, this file is using CRLF, like all other random files I tested in endpoints/lib so this doesn't seem to be an issue that needs to be corrected.

Testers, go ahead and see if this works. I will join you guys shortly and update to VirtualBox 6.1. This PR has been updated to work with the current develop branch as of January 29, 2020.
This zip file has both: https://github.com/chotaire/phpvirtualbox/archive/chot-nested-virtualization.zip

I installed vbox 6.1 and tried branch "develop", but when I click on my VM I get an "Attribute does not exist" error:

`
Exception Object
(
[message:protected] => Attribute does not exist
[string:Exception:private] =>
[code:protected] => 0
[file:protected] => /usr/local/share/phpvirtualbox-git/endpoints/lib/vboxServiceWrappers.php
[line:protected] => 56
[trace:Exception:private] => Array
(
[0] => Array
(
[file] => /usr/local/share/phpvirtualbox-git/endpoints/lib/vboxconnector.php
[line] => 4225
[function] => __get
[class] => VBox_ManagedObject
[type] => ->
[args] => Array
(
[0] => VRAMSize
)

            )

        [1] => Array
            (
                [file] => /usr/local/share/phpvirtualbox-git/endpoints/lib/vboxconnector.php
                [line] => 3567
                [function] => _machineGetDetails
                [class] => vboxconnector
                [type] => ->
                [args] => Array
                    (
                        [0] => IMachine Object
                            (
                                [connection:protected] => SoapClient Object
                                    (
                                        [location] => http://127.0.0.1:18083/
                                        [_features] => 5
                                        [_connection_timeout] => 20
                                        [_stream_context] => Resource id #5
                                        [_soap_version] => 1
                                        [sdl] => Resource id #6
                                        [httpsocket] => Resource id #7
                                        [_use_proxy] => 0
                                        [httpurl] => Resource id #23
                                    )

                                [handle:protected] => b826cfc00a473451-0000000000000002
                            )

                    )

            )

        [2] => Array
            (
                [file] => /usr/local/share/phpvirtualbox-git/endpoints/lib/vboxconnector.php
                [line] => 951
                [function] => remote_machineGetDetails
                [class] => vboxconnector
                [type] => ->
                [args] => Array
                    (
                        [0] => Array
                            (
                                [vm] => 67052513-29c0-4e15-815b-a7692cac84b4
                            )

                    )

            )

        [3] => Array
            (
                [file] => /usr/local/share/phpvirtualbox-git/endpoints/api.php
                [line] => 316
                [function] => __call
                [class] => vboxconnector
                [type] => ->
                [args] => Array
                    (
                        [0] => machineGetDetails
                        [1] => Array
                            (
                                [0] => Array
                                    (
                                        [vm] => 67052513-29c0-4e15-815b-a7692cac84b4
                                    )

                                [1] => Array
                                    (
                                        [0] => Array
                                            (
                                                [data] => Array
                                                    (
                                                        [responseData] => Array
                                                            (
                                                            )

                                                    )

                                                [errors] => Array
                                                    (
                                                    )

                                                [persist] => Array
                                                    (
                                                    )

                                                [messages] => Array
                                                    (
                                                    )

                                            )

                                    )

                            )

                    )

            )

    )

[previous:Exception:private] => 

)
`

@Polve Yep it is caused by a different PR. I am currently working on a fix. Gotta go shopping quickly then I will supply a patch.

@Polve Please try the supplied fix. Thanks @cesium147 for the VirtualBox-6.1 patch.
Updated download: https://github.com/chotaire/phpvirtualbox/archive/chot-nested-virtualization.zip (current develop branch plus this PR).

After upgrading to VirtualBox 6.1, I've tried pretty much all features, creating, editing, deleting VMs and playing with most configuration parameters, creating, restoring, merging, deleting snapshots, changing VGA adapters (haven't tested if they actually work), setting Nested Virtualization feature on/off, installing guest additions. Even configuring multiple concurrent connections in remote display now works. From what I can see, everything works like a charme. This is awesome.

The only feature I haven't tried is Shared Folders.

@h6w Please review with most urgent priority as it fixes an issue that all VirtualBox 6.1 users will experience on current develop branch (throwing a fatal exception when simply clicking a VM). Once this is merged, VirtualBox 6.1 users should have a much more stable phpvirtualbox than 6.0 users ever had.

@Polve Please try the supplied fix

It works fine for me, thanks!

Ok, so we're going back to the $m->GraphicsAdapter model. I must admit that raised an eyebrow when it was changed the first time. This seems a more sensible model. Thanks! :-)

@h6w thanks for the quick action. I hope this will give developers a good base to work with in the future as we now have a really well working 6.1 version. I've tested it a lot today and decided to move it to production, it works so much better than master branch.

VirtualBox 5.2 goes out of support in July 2020. I hope you can keep an eye on this and merge develop to master in time. That would also stop confusing users wondering why there is no support for 6.0 and 6.1.

All eyes on @pasha1st and @cesium147 who made this huge step forward possible. ♥

Yep. Let's give it 14 days to settle in and then I'll release if there aren't any more reports of major issues. :-)

The VirtualBox dev team in Germany being thankful for the news is sure "that it will make people happy".