virtio-gpu: split virtio-gpu-pci & virtio-vga

Add base classes that are common to vhost-user-gpu-pci and
vhost-user-vga.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190524130946.31736-9-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>