Remove fakelibvirt

The tests currently use real `libvirt` if it's installed and falls back to `fakelibvirt` if not. If there's a divergence between the two, this can lead to a situation where the tests pass locally but fail when run on Jenkins.

The solution is to use real `libvirt` if available, and skip the tests if not (as we do with the DB-migration-tests if a particular DB driver isn't present).