libvirt: Uses correct imagebackend for configdrive

When the configdrive file is created it must be moved to the
configured imagebackend, not always the raw one.

Otherwise nova can't boot an instance with a configdrive attached
when the rbd backend is configured.

Related to bug #1246201
Closes-bug: #1361840

Change-Id: I59578d9b87a36ff4417a30ff14c5774c54698cf3
Merge "libvirt: Uses correct imagebackend for configdrive"