Make begin_detaching fail if volume not "in-use"

Like it's counterpart from Nova's volume-attach functionality standpoint
- reserve_volume, begin_detaching should fail if the volume is not in
  the correct state to be detached. This will prevent nova from
attempting to check and then detach, which is inherently racy.

Change-Id: Ie87eb0c9aea068affc94032ab2e53a91888d272a
Partial-bug: #1302774
Merge "Make begin_detaching fail if volume not "in-use""