Commit ecd8725c dropped attempts to probe the cgconfig service on
new enough Fedora where systemd took over that aspect of the system,
but mistakenly used F14 instead of F15 as the cutoff point.

https://bugzilla.redhat.com/show_bug.cgi?id=741358

Also, RHEL does not include HyperV support yet.

* libvirt.spec.in (with_cgconfig): Check cgconfig service in F15.
(%{?rhel}): Provide default for with_hyperv.