Compute VM power usage #168

Host power consumption is computed based on VM CPU utilization history, as presented in some examples such as the PowerExample.java.

However, it's required a big effort to correctly compute the power consumption share for an isolated VM.

New methods should be introduced to the UtilizationHistory interface to enable computing the:

• VM's power consumption relative to the Host's power consumption;
• VM's CPU usage relative to the Host's total CPU capacity (total MIPS).