nixos/test-driver: fix resource cleanup of vlan/qmp objects

Using __del__ is somewhat unsound resource cleanup in our clase the
logger already closed its logfile and therefor fails with exception
before the rest of the resources can be cleaned up.

nixos/lib/test-driver/test_driver/machine.py

@@ -1234,6 +1234,9 @@ class Machine:
         self.monitor.close()
         self.serial_thread.join()
 
+        if self.qmp_client:
+            self.qmp_client.close()
+
     def run_callbacks(self) -> None:
         for callback in self.callbacks:
             callback()