The _image_create() code in compute manager is the only place where
db.instance_test_and_set is used.. which was added before
'expected_task_state' support was added to instance_update. By
switching to use self.update(), we can also take advantage of it doing
the state change notification for us.
The instance update and the compute RPC call have been moved out of
_create_image() and into snapshot() and backup() to support a future
cells patch.
Change-Id: Ib6603f732d28e49cb6f351447da5f6e41615dea5
7f83901b36