Fix virbr0 elimination on compute nodes

https://review.openstack.org/41319 added some code to remove the
default virbr0 bridge created by libvirt. However, it requires
the libvirtd service to be up, not just installed. Thus, it was
not working as expected.

Change-Id: Idf82db9ede802d9875aab142db596ee544aadd97