Add support for cursor dmabufs.  qemu has to render the cursor for
that, so in case a cursor is present qemu allocates a new dmabuf, blits
the scanout, blends in the pointer and passes on the new dmabuf to
spice-server.  Without cursor qemu continues to simply pass on the
scanout dmabuf as-is.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180308090618.30147-4-kraxel@redhat.com