[Libvir] Support graphical console for PV Xen guests

With the paravirt framebuffer patches for Xen, we can support having a graphical console for PV as well as HVM guests. The attached adds support to libvirt by breaking out the <graphics> parsing into its own function that can be used by both PV and HVM guests. It also adds support for looking at the appropriate sexpr for the vnc/sdl console for PV guests. One thing that is potentially not okay is that I've changed sexpr_node to take a format string instead of just a const char *, but since it's not public API and doesn't actually change anything for callers I think it should be fine. If not, let me know and I can change it. Jeremy