This commit updates the volume orchestration to lock on the volume ID
before sending the attach commands. While there is already validation
for checking if a VM is already attached to an instance in both the
API command and the volume orchestration framework, they aren't of
much use if there's no synchronization.
With this commit, a single volume being attached to multiple instances
will successfully attach to one and return the error added in commit
8aa34d0 for the other requests.