vhost-user: add vhost_user_gpu_set_socket()

Add a new vhost-user message to give a unix socket to a vhost-user
backend for GPU display updates.

Back when I started that work, I added a new GPU channel because the
vhost-user protocol wasn't bidirectional. Since then, there is a
vhost-user-slave channel for the slave to send requests to the master.
We could extend it with GPU messages. However, the GPU protocol is
quite orthogonal to vhost-user, thus I chose to have a new dedicated
channel.

See vhost-user-gpu.rst for the protocol details.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190524130946.31736-2-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

docs/interop/vhost-user.rst

@@ -1163,6 +1163,15 @@ Master message types
   send the shared inflight buffer back to slave so that slave could
   get inflight I/O after a crash or restart.
 
+``VHOST_USER_GPU_SET_SOCKET``
+  :id: 33
+  :equivalent ioctl: N/A
+  :master payload: N/A
+
+  Sets the GPU protocol socket file descriptor, which is passed as
+  ancillary data. The GPU protocol is used to inform the master of
+  rendering state and updates. See vhost-user-gpu.rst for details.
+
 Slave message types
 -------------------