qemu: Fix crash during qemuStateCleanup

Rather than waiting until we've free'd up all the resources, cause the
'workerPool' thread pool to flush as soon as possible during stateCleanup.
Otherwise, it's possible something waiting to run will SEGV such as is the
case during race conditions of simultaneous exiting libvirtd and qemu process.
Resolves the following crash:

[1] crash backtrace: (bt is shortened a bit):

0  0x00007ffff7282f2b in virClassIsDerivedFrom
   (klass=0xdeadbeef, parent=0x55555581d650) at util/virobject.c:169
1  0x00007ffff72835fd in virObjectIsClass
   (anyobj=0x7fffd024f580, klass=0x55555581d650) at util/virobject.c:365
2  0x00007ffff7283498 in virObjectLock
   (anyobj=0x7fffd024f580) at util/virobject.c:317
3  0x00007ffff722f0a3 in virCloseCallbacksUnset
   (closeCallbacks=0x7fffd024f580, vm=0x7fffd0194db0,
    cb=0x7fffdf1af765 <qemuProcessAutoDestroy>)
   at util/virclosecallbacks.c:164
4  0x00007fffdf1afa7b in qemuProcessAutoDestroyRemove
   (driver=0x7fffd00f3a60, vm=0x7fffd0194db0) at qemu/qemu_process.c:6365
5  0x00007fffdf1adff1 in qemuProcessStop
   (driver=0x7fffd00f3a60, vm=0x7fffd0194db0, reason=VIR_DOMAIN_SHUTOFF_CRASHED,
    asyncJob=QEMU_ASYNC_JOB_NONE, flags=0)
   at qemu/qemu_process.c:5877
6  0x00007fffdf1f711c in processMonitorEOFEvent
   (driver=0x7fffd00f3a60, vm=0x7fffd0194db0) at qemu/qemu_driver.c:4545
7  0x00007fffdf1f7313 in qemuProcessEventHandler
   (data=0x555555832710, opaque=0x7fffd00f3a60) at qemu/qemu_driver.c:4589
8  0x00007ffff72a84c4 in virThreadPoolWorker
   (opaque=0x555555805da0) at util/virthreadpool.c:167

Thread 1 (Thread 0x7ffff7fb1880 (LWP 494472)):
1  0x00007ffff72a7898 in virCondWait
   (c=0x7fffd01c21f8, m=0x7fffd01c21a0) at util/virthread.c:154
2  0x00007ffff72a8a22 in virThreadPoolFree
   (pool=0x7fffd01c2160) at util/virthreadpool.c:290
3  0x00007fffdf1edd44 in qemuStateCleanup ()
   at qemu/qemu_driver.c:1102
4  0x00007ffff736570a in virStateCleanup ()
   at libvirt.c:807
5  0x000055555556f991 in main (argc=1, argv=0x7fffffffe458) at libvirtd.c:1660

src/qemu/qemu_driver.c

@@ -1070,6 +1070,7 @@ qemuStateCleanup(void)
         return -1;
 
     virNWFilterUnRegisterCallbackDriver(&qemuCallbackDriver);
+    virThreadPoolFree(qemu_driver->workerPool);
     virObjectUnref(qemu_driver->config);
     virObjectUnref(qemu_driver->hostdevMgr);
     virHashFree(qemu_driver->sharedDevices);
@@ -1100,7 +1101,6 @@ qemuStateCleanup(void)
     virLockManagerPluginUnref(qemu_driver->lockManager);
 
     virMutexDestroy(&qemu_driver->lock);
-    virThreadPoolFree(qemu_driver->workerPool);
     VIR_FREE(qemu_driver);
 
     return 0;