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).

Fixes: bug #1300788

(cherry picked from commit 98fb5e56ef96c0c6a8754276e5f8c9c0b7e50fed)

Change-Id: I1b95caec2dad6580db576aa6f80f47913b7c066e
Merge "VMware: validate that VM exists on backend prior to deletion" into stable/icehouse