VHOST_USER_GPU_DMABUF_SCANOUT2 is defined as a message with all the
contents of VHOST_USER_GPU_DMABUF_SCANOUT plus the dmabuf modifiers
which were ommitted.

The VHOST_USER_GPU_PROTOCOL_F_DMABUF2 protocol feature is defined as a
way to check whether this new message is supported or not.

Signed-off-by: Erico Nunes <ernunes@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-Id: <20230714153900.475857-2-ernunes@redhat.com>