This completes the implementation on the libguestfs side, allowing
python-libvirt dom pointers to be passed to guestfs_add_libvirt_dom.
For context see:
https://bugzilla.redhat.com/show_bug.cgi?id=1138203#c40
https://bugzilla.redhat.com/show_bug.cgi?id=1075143
https://bugzilla.redhat.com/show_bug.cgi?id=1075164
Rich.