contrib/vhost-user-gpu: implement get_edid feature

contrib/vhost-user-gpu: implement get_edid feature

Implement the virtio-gpu feature in contrib/vhost-user-gpu, which was
unsupported until now.
In this implementation, the feature is enabled inconditionally to avoid
creating another optional config argument.
Similarly to get_display_info, vhost-user-gpu sends a message back to
the frontend to have access to all the display information. In the
case of get_edid, it also needs to pass which scanout we should
retrieve the edid for.

The VHOST_USER_GPU_PROTOCOL_F_EDID protocol feature is required if the
frontend sets the VIRTIO_GPU_F_EDID virtio-gpu feature. If the frontend
sets the virtio-gpu feature but does not support the protocol feature,
the backend will abort with an error.

Signed-off-by: Erico Nunes <ernunes@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230626164708.1163239-4-ernunes@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>