cpus: flush all requests on each vm stop

Flush all requests once we have stopped all
cpus and devices.
Make sure disk is in consistent state.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Jason Wang <jasowang@redhat.com>
Acked-by: Marcelo Tosatti <mtosatti@redhat.com>

cpus.c

@@ -111,6 +111,8 @@ static void do_vm_stop(int reason)
         vm_running = 0;
         pause_all_vcpus();
         vm_state_notify(0, reason);
+        qemu_aio_flush();
+        bdrv_flush_all();
         monitor_protocol_event(QEVENT_STOP, NULL);
     }
 }