Fixes live migration with attached volumes issue

Fixes Bug: 1140917

Live migration fails when there are volumes attached to the instance

Change-Id: I27b39ac41e97840d2a60006b1c74f6d256bc53b7
Fix an error in compute api snapshot_volume_backed bdm code

Stumbled upon an error similar to the one found in
https://review.openstack.org/#/c/23354/ where a dbm dict
item is tried to be read as an attribute.

Resolves bug 1142734.

Change-Id: Ie639cd483334b6222085a139159da47ce6fa3671