vconn: Convert vconn code to modern OVS structure.

The vconn code is a relative fossil as OVS code goes.  It was written
before we had really figured how code should fit together.  Part of that
history is that it used poll_fd_callback() to register callbacks without
the assistance of other code.  That isn't how the rest of OVS works now;
this code is the only remaining user of that function.

To make it more like the rest of the system, this code gets rid of the use
of poll_fd_callback().  It also adds vconn_run() and vconn_run_wait()
functions and calls to them from the places where they are now required.

lib/vconn-unix.c

@@ -71,6 +71,8 @@ struct vconn_class unix_vconn_class = {
     NULL,                       /* connect */
     NULL,                       /* recv */
     NULL,                       /* send */
+    NULL,                       /* run */
+    NULL,                       /* run_wait */
     NULL,                       /* wait */
 };