Do not instantiate an extra default VGA device if -device virtio-vga-gl
is provided.

Related to commit b36eb886 ("virtio-gpu:
add virtio-vga-gl")

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210701062421.721414-1-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>