unlock the monitor when unwatching the monitor

Steps to reproduce this bug:
# virsh qemu-monitor-command domain 'cpu_set 2 online' --hmp
The domain has 2 cpus, and we try to set the third cpu online.
The qemu crashes, and this command will hang.

The reason is that the refs is not 1 when we unwatch the monitor.
We lock the monitor, but we do not unlock it. So virCondWait()
will be blocked.
Signed-off-by: NWen Congyang <wency@cn.fujitsu.com>

src/qemu/qemu_monitor.c

@@ -239,7 +239,8 @@ qemuMonitorUnwatch(void *monitor)
     qemuMonitorPtr mon = monitor;
 
     qemuMonitorLock(mon);
-    qemuMonitorUnref(mon);
+    if (qemuMonitorUnref(mon) > 0)
+        qemuMonitorUnlock(mon);
 }
 
 static int