qemu: Avoid loop of fake reboots

Once virDomainReboot is called for a domain, guest OS initiated shutdown
would always result in reboot instead of shutdown. Only
virDomainShutdown would actually shutd such domain down. That's because
we forgot to reset fakeReboot flag once we asked the domain to reboot.

src/qemu/qemu_process.c

@@ -445,6 +445,7 @@ qemuProcessHandleShutdown(qemuMonitorPtr mon ATTRIBUTE_UNUSED,
 
     priv->gotShutdown = true;
     if (priv->fakeReboot) {
+        priv->fakeReboot = false;
         virDomainObjRef(vm);
         virThread th;
         if (virThreadCreate(&th,