monitor: cleanup detection of qmp_dispatcher_co shutting down

Instead of overloading qmp_dispatcher_co_busy, make the coroutine
pointer NULL.  This will make things break spectacularly if somebody
tries to start a request after monitor_cleanup().

AIO_WAIT_WHILE_UNLOCKED() does not need qatomic_mb_read(), because
the macro contains all the necessary memory barriers.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

monitor/monitor.c

@@ -686,7 +686,7 @@ void monitor_cleanup(void)
 
     AIO_WAIT_WHILE_UNLOCKED(NULL,
                    (aio_poll(iohandler_get_aio_context(), false),
-                    qatomic_mb_read(&qmp_dispatcher_co_busy)));
+                    qatomic_read(&qmp_dispatcher_co)));
 
     /*
      * We need to explicitly stop the I/O thread (but not destroy it),

monitor/qmp.c

@@ -226,6 +226,7 @@ void coroutine_fn monitor_qmp_dispatcher_co(void *data)
 
         /* On shutdown, don't take any more requests from the queue */
         if (qmp_dispatcher_co_shutdown) {
+            qatomic_set(&qmp_dispatcher_co, NULL);
             return;
         }
 
@@ -250,6 +251,7 @@ void coroutine_fn monitor_qmp_dispatcher_co(void *data)
              * yielded and were reentered from monitor_cleanup()
              */
             if (qmp_dispatcher_co_shutdown) {
+                qatomic_set(&qmp_dispatcher_co, NULL);
                 return;
             }
         }