docs: vhost-user: replace master/slave with front-end/back-end

This matches the nomenclature that is generally used.  Also commonly used
is client/server, but it is not as clear because sometimes the front-end
exposes a passive (server) socket that the back-end connects to.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210226143413.188046-4-pbonzini@redhat.com>
Message-Id: <20220321153037.3622127-7-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/interop/vhost-user-gpu.rst

@@ -13,10 +13,10 @@ Introduction
 ============
 
 The vhost-user-gpu protocol is aiming at sharing the rendering result
-of a virtio-gpu, done from a vhost-user slave process to a vhost-user
-master process (such as QEMU). It bears a resemblance to a display
+of a virtio-gpu, done from a vhost-user back-end process to a vhost-user
+front-end process (such as QEMU). It bears a resemblance to a display
 server protocol, if you consider QEMU as the display server and the
-slave as the client, but in a very limited way. Typically, it will
+back-end as the client, but in a very limited way. Typically, it will
 work by setting a scanout/display configuration, before sending flush
 events for the display updates. It will also update the cursor shape
 and position.
@@ -26,8 +26,8 @@ socket ancillary data to share opened file descriptors (DMABUF fds or
 shared memory). The socket is usually obtained via
 ``VHOST_USER_GPU_SET_SOCKET``.
 
-Requests are sent by the *slave*, and the optional replies by the
-*master*.
+Requests are sent by the *back-end*, and the optional replies by the
+*front-end*.
 
 Wire format
 ===========