[Libguestfs] RHN support and capabilities

This patch series includes a repost of Milan's unmodified RHN support patch because I haven't pushed it yet. On top of that patch, it includes the capabilities patch in as many bits as I could make it into. The big one is 7/8. I've tested all of the following guests both with and without RHN registration: Xen RHEL 54 PV Xen RHEL 51 PV Xen RHEL 48 PV ESX RHEL 54 FV