Use different libvirt drivers for launch and migration.

When doing a pure launch we want to restrict the possible image
backends to only qcow2. To support this a libvirt driver is created
then modified so that its image backend will only return qcow2. For
migration we want to support the full list of available backends.

Change-Id: Ifec60b1be12aba9c2ce8973d71f7a989b7f6b9bc