virtio-gpu: add live migration support

Store some additional state for cursor and resource backing storage,
so we can write out and reload things.  Implement vmsave+vmload for
2d mode.  Continue blocking live migration in 3d/virgl mode.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1464009727-7753-1-git-send-email-kraxel@redhat.com