Shelve: The snapshot should be removed when delete instance.

When deleting a vm with state "shelved" or "shelved-offloaded",
the vm's snapshot like ‘shelved image’ should also be removed.

Change-Id: Ie9b0f8dec8f85351ef188f70e0fe9414e332aafe
Closes-Bug: #1259462
Merge "Shelve: The snapshot should be removed when delete instance."