libguestfs appears to be the best way to access images for injection,
so add the python bindings to the RPM list. Indeed, on RHEL it is the
only way, because the fallback of nbd access isn't available. There,
this change prevents a lot of errors in nova.virt.disk.vfs.localfs
when it can't import the nbd module.
Change-Id: I9c2a81bb7c26bc17f8b5f1b5a682c05a419b33ba
4ca55309f2