VMware: power_off_instance support

Commit 1e1915aaaca38b5691794e0e052a42b9d95dd3c2 introduced a new
utility method to shutdown a VM. This will now be moved to vm_util
similar to refactoring in blueprint vmware-spawn-refactor.

This patch does the following:
1. Creates a vm_util method - power_off_instance.
2. Corrects the unit test for the unrescue. That did not correctly validate
   the correct vm ref for the power off
3. Added specific tests for the new method that has been added
4. Drops the test test_power_off_suspended as this is not a valid test
   case.

Change-Id: I6b7c260488bbd0278f6e7ce98be77e958f1576ab
Merge "VMware: power_off_instance support"