docs: vhost-user: add Virtio status protocol feature

docs: vhost-user: add Virtio status protocol feature

This patch specifies the VHOST_USER_SET_STATUS and
VHOST_USER_GET_STATUS requests, which are sent by
the master to update and query the Virtio status
in the backend.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Message-Id: <20200618134501.145747-1-maxime.coquelin@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>