Fix detach twice of snapshot
Fix detach twice of snapshot

The temp snapshot used for backing-up the in-use volume is detached twice.
The second time an exception is thrown saying the device not found.

Change-Id: I6eb237c753cefefecbdf8bc2a517f5eda5d40862
Closes-Bug: 1649792

j-griffith/cinder,ge0rgi/cinder,eharney/cinder,mahak/cinder,Datera/cinder,j-griffith/cinder,phenoxim/cinder,openstack/cinder,openstack/cinder,Datera/cinder,eharney/cinder,phenoxim/cinder,mahak/cinder