contrib: add vhost-user-gpu

Add a vhost-user gpu backend, based on virtio-gpu/3d device. It is
associated with a vhost-user-gpu device.

Various TODO and nice to have items:
- multi-head support
- crash & resume handling
- accelerated rendering/display that avoids the waiting round trips
- edid support

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190524130946.31736-6-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>