qemuProcessEventSubmit : Unref event->vm instead of vm

In error case, unref event->vm instead of vm. This makes it
easier for the reader to understand as it is the event struct
that's holding the reference.

Signed-off-by: Shaleen Bathla <shaleen.bathla@oracle.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_process.c

@@ -287,7 +287,7 @@ qemuProcessEventSubmit(virDomainObj *vm,
     event->data = data;
 
     if (virThreadPoolSendJob(driver->workerPool, 0, event) < 0) {
-        virObjectUnref(vm);
+        virObjectUnref(event->vm);
         qemuProcessEventFree(event);
     }
 }