libvirt-devel contains libvirt.pc, but it does not have a dependency on pkgconfig. There was a thread on fedora-packaging about this a couple of months ago, and the best reason for including the hard dependency was that otherwise the pkgconfig directory could be un-owned. http://www.redhat.com/archives/fedora-packaging/2006-June/msg00178.html
Created attachment 134090 [details] spec patch
It looks like libvirt-devel is also missing a dependency on xen-devel as well.
Created attachment 134472 [details] Updated spec patch
Okay, agreed except the Requirement on xen-devel should not be tied to a specific version, applied, pushing it, it should be fixed in Rawhide soonish, thanks Daniel