qemu: avoid double shutdown

* src/qemu/qemu_driver.c (qemudShutdownVMDaemon): Check that vm is
still active.
Reported by Wen Congyang as follows:

Steps to reproduce this bug:

1. use gdb to debug libvirtd, and set breakpoint in the function
   qemuConnectMonitor()
2. start a vm, and the libvirtd will be stopped in qemuConnectMonitor()
3. kill -STOP $(cat /var/run/libvirt/qemu/<domain>.pid)
4. continue to run libvirtd in gdb, and libvirtd will be blocked in the
   function qemuMonitorSetCapabilities()
5. kill -9 $(cat /var/run/libvirt/qemu/<domain>.pid)

Here is log of the qemu:
=========
LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin ...
char device redirected to /dev/pts/3
2011-01-27 09:38:48.101: shutting down
2011-01-27 09:41:26.401: shutting down
=========

The vm is shut down twice. I do not know whether this behavior has
side effect, but I think we should shutdown the vm only once.

src/qemu/qemu_driver.c

@@ -2991,6 +2991,11 @@ static void qemudShutdownVMDaemon(struct qemud_driver *driver,
     VIR_DEBUG("Shutting down VM '%s' pid=%d migrated=%d",
               vm->def->name, vm->pid, migrated);
 
+    if (!virDomainObjIsActive(vm)) {
+        VIR_DEBUG("VM '%s' not active", vm->def->name);
+        return;
+    }
+
     if ((logfile = qemudLogFD(driver, vm->def->name, true)) < 0) {
         /* To not break the normal domain shutdown process, skip the
          * timestamp log writing if failed on opening log file. */