spec: prefer canonical name of util-linux

spec: prefer canonical name of util-linux

I noticed that in two places, we require util-linux, and in a third,
we require util-linux-ng.  On Fedora (I tested F15 through rawhide),
util-linux-ng is obsoleted by util-linux; on RHEL 6, util-linux
is obsoleted by util-linux-ng.  That is, on either platform, either
name will get you the correct package installed (where the preferred
name on fedora is util-linux, and on RHEL 6 is util-linux-ng).  But
on RHEL 5, there is no util-linux-ng

* libvirt.spec.in (Requires): Use util-linux, not util-linux-ng.