conf: Add a new 'xenbus' controller type

conf: Add a new 'xenbus' controller type

xenbus is virtual controller (akin to virtio controllers) for Xen
paravirtual devices. Although all Xen VMs have a xenbus, it has
never been modeled in libvirt, or in Xen native VM config format
for that matter.

Recently there have been requests to support Xen's max_grant_frames
setting in libvirt. max_grant_frames is best modeled as an attribute
of xenbus. It describes the maximum IO buffer space (or DMA space)
available in xenbus for use by connected paravirtual devices. This
patch introduces a new xenbus controller type that includes a
maxGrantFrames attribute.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>