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
Fixed the comment spelling error - voumes to volumes

Closes-bug: #1325481
Change-Id: I38a1d9df98c709d3ca173fa61315ad3f7ca9ffdc
Merge "Fixed the comment spelling error - voumes to volumes"
Merge "Make begin_detaching fail if volume not "in-use""