xenopsd: Uncomment the libvirt subpackage

We stopped building xenopsd-libvirt in change 3ce7fb70 (in fact the package still existed, but it was empty).   We forgot to remove its %post directive, causing a problem that was reported on the xen-api mailing list: 

> http://lists.xen.org/archives/html/xen-api/2013-08/msg00069.html