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.

Looked at the next couple of lines and discovered more
cases of this.

Updates to some api tests that contain fakes that hide this
error adds some coverage for this error. Added a dependancy
to the above review to be able to change the tests, since
the test changes hits both of these errors.

Resolves bug 1142734.

Change-Id: Ie639cd483334b6222085a139159da47ce6fa3671