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>

docs/interop/vhost-user.rst

@@ -816,6 +816,7 @@ Protocol features
   #define VHOST_USER_PROTOCOL_F_RESET_DEVICE         13
   #define VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS 14
   #define VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS  15
+  #define VHOST_USER_PROTOCOL_F_STATUS               16
 
 Master message types
 --------------------
@@ -1307,6 +1308,29 @@ Master message types
   ``VHOST_USER_ADD_MEM_REG`` message, this message is used to set and
   update the memory tables of the slave device.
 
+``VHOST_USER_SET_STATUS``
+  :id: 39
+  :equivalent ioctl: VHOST_VDPA_SET_STATUS
+  :slave payload: N/A
+  :master payload: ``u64``
+
+  When the ``VHOST_USER_PROTOCOL_F_STATUS`` protocol feature has been
+  successfully negotiated, this message is submitted by the master to
+  notify the backend with updated device status as defined in the Virtio
+  specification.
+
+``VHOST_USER_GET_STATUS``
+  :id: 40
+  :equivalent ioctl: VHOST_VDPA_GET_STATUS
+  :slave payload: ``u64``
+  :master payload: N/A
+
+  When the ``VHOST_USER_PROTOCOL_F_STATUS`` protocol feature has been
+  successfully negotiated, this message is submitted by the master to
+  query the backend for its device status as defined in the Virtio
+  specification.
+
+
 Slave message types
 -------------------