Shutdown of already shut down VM need a better error message

[pevogam]

Trying to shut down an already shut down VM results in this:

"ERROR: argument of type 'NoneType' is not iterable"

Gory details:
[stderr] Traceback (most recent call last):
[stderr] File "/usr/lib/python3.10/site-packages/avocado_vt/plugins/vt_runner.py", line 71, in runTest
[stderr] raise self.__status # pylint: disable-msg=E0702
[stderr] File "/usr/lib/python3.10/site-packages/avocado_vt/test.py", line 146, in setUp
[stderr] self._runTest()
[stderr] File "/usr/lib/python3.10/site-packages/avocado_vt/test.py", line 264, in _runTest
[stderr] run_func(self, params, env)
[stderr] File "/usr/lib/python3.10/site-packages/virttest/error_context.py", line 135, in new_fn
[stderr] return fn(*args, **kwargs)
[stderr] File "/mnt/local/intra2net-qa/tp_i2n/tests/shared_manage_vm.py", line 92, in run
[stderr] vmnet.start_all_sessions()
[stderr] File "/usr/lib/python3.10/site-packages/avocado_i2n/vmnet/network.py", line 138, in start_all_sessions
[stderr] node.get_session()
[stderr] File "/usr/lib/python3.10/site-packages/avocado_i2n/vmnet/node.py", line 161, in get_session
[stderr] self.platform.verify_alive()
[stderr] File "/usr/lib/python3.10/site-packages/virttest/qemu_vm.py", line 298, in verify_alive
[stderr] self.verify_userspace_crash()
[stderr] File "/usr/lib/python3.10/site-packages/virttest/qemu_vm.py", line 362, in verify_userspace_crash
[stderr] if "(core dumped)" in self.process.get_output():
[stderr] TypeError: argument of type 'NoneType' is not iterable