VMWare: add power off vm before detach disk during unrescue

Non Hot Plug type disk like IDE can only be detached when the VM is power off.

Change-Id: Ib1f387a41abe2b52357854e90c2535ebb7b43f18
Close-bug: #1279199
Merge "VMWare: add power off vm before detach disk during unrescue"