The user explicitly requested spice GL, so if we know it isn't
going to work we should exit

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: e3789e35b16f9e3cc6f2652f91c52d88ba6d6936.1463588606.git.crobinso@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>