VMware: validate that VM exists on backend prior to deletion

Attempting to delete an instance that does not exist on the backend
throws an exception (for example UnregisterVM may return this when
running the CI).

Change-Id: I1b95caec2dad6580db576aa6f80f47913b7c066e
Closes-bug: #1300788
Merge "VMware: validate that VM exists on backend prior to deletion"