Add xml to the private data for a disk source to represent the nbdkit
process so that the state can be re-created if the libvirt daemon is
restarted. Format:
<nbdkit>
<pidfile>/path/to/nbdkit.pid</pidfile>
<socketfile>/path/to/nbdkit.socket</socketfile>
</nbdkit>
Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>