virtio-gpu: check max_outputs value

The value must be less than VIRTIO_GPU_MAX_SCANOUT.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1463653560-26958-4-git-send-email-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
