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
(cherry picked from commit 1e1915aaaca38b5691794e0e052a42b9d95dd3c2)
Merge "VMWare: add power off vm before detach disk during unrescue" into stable/icehouse