Allow VM resource's power_state to be managed

[ddelnano]

Summary: Allow VM resource's power_state to be managed

The VM resource's previous power_state attribute was computed and didn't allow for specifying the state as an input. This change implements the necessary support for creating VMs with a non default power_state and to manage the lifecycle of it as needed and completes #263.

Testing Done: The following checks were successful

• New tests verifies that the new functionality works as expected for creating and updating VMs
• Acceptance tests pass

Todo:

• Clean up vm power state options (remove hardcoding) and remove bogus values ("Stopped" isn't a real state)
• Add support for suspend and pause
• Prevent destroy_cloud_config_vdi_after_boot = true and a non running power_state from being valid configuration