do not unref obj in qemuDomainObjExitMonitor*

Steps to reproduce this bug:
# cat test.sh
  #! /bin/bash -x
  virsh start domain
  sleep 5
  virsh qemu-monitor-command domain 'cpu_set 2 online' --hmp
# while true; do ./test.sh ; done

Then libvirtd will crash.

The reason is that:
we add a reference of obj when we open the monitor. We will reduce this
reference when we free the monitor.

If the reference of monitor is 0, we will free monitor automatically and
the reference of obj is reduced.

But in the function qemuDomainObjExitMonitorWithDriver(), we reduce this
reference again when the reference of monitor is 0.

It will cause the obj be freed in the function qemuDomainObjEndJob().

Then we start the domain again, and libvirtd will crash in the function
virDomainObjListSearchName(), because we pass a null pointer(obj->def->name)
to strcmp().
Signed-off-by: NWen Congyang <wency@cn.fujitsu.com>

src/qemu/qemu_domain.c

@@ -588,7 +588,6 @@ void qemuDomainObjExitMonitor(virDomainObjPtr obj)
     virDomainObjLock(obj);
 
     if (refs == 0) {
-        virDomainObjUnref(obj);
         priv->mon = NULL;
     }
 }
@@ -634,7 +633,6 @@ void qemuDomainObjExitMonitorWithDriver(struct qemud_driver *driver,
     virDomainObjLock(obj);
 
     if (refs == 0) {
-        virDomainObjUnref(obj);
         priv->mon = NULL;
     }
 }