vhost-user: more master/slave things

we switched to front-end/back-end, but newer patches
reintroduced old language. Fix this up.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/interop/vhost-user.rst

@@ -340,7 +340,7 @@ drivers cannot negotiate it.
 
 This reserved feature bit was reused by the vhost-user protocol to add
 vhost-user protocol feature negotiation in a backwards compatible
-fashion. Old vhost-user master and slave implementations continue to
+fashion. Old vhost-user front-end and back-end implementations continue to
 work even though they are not aware of vhost-user protocol feature
 negotiation.