hw/display: add vhost-user-vga & gpu-pci

Add new virtio-gpu devices with a "vhost-user" property. The
associated vhost-user backend is used to handle the virtio rings and
provide rendering results thanks to the vhost-user-gpu protocol.

Example usage:
-object vhost-user-backend,id=vug,cmd="./vhost-user-gpu"
-device vhost-user-vga,vhost-user=vug

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190524130946.31736-10-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>