qtwrapper: Wait for all threads to finish

Call DRing::fini(), just like the dbus client does

Refs #70803

src/qtwrapper/instancemanager_wrap.h

@@ -54,6 +54,7 @@ public Q_SLOTS: // METHODS
    void Unregister(int pid)
    {
       Q_UNUSED(pid) //When directly linked, the PID is always the current process PID
+      DRing::fini();
    }
 
    bool isConnected();