virttest: handle failed qmp_cmds without crashing

[bssrikanth]

get_qmp_cmds could return None when the query-commands probe produced no parseable output, so has_qmp_cmd did cmd in None and raised TypeError.

Always return a list (empty on failure), log a warning, and treat an empty command set as unsupported so capability checks return false instead of crashing.

[bssrikanth]

Before fix:

..
[stdlog] Traceback (most recent call last):
[stdlog]   File "/usr/local/lib/python3.12/dist-packages/virttest/qemu_vm.py", line 3774, in create
[stdlog]     self.devices, self.spice_options = self.make_create_command()
[stdlog]                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
[stdlog]   File "/usr/local/lib/python3.12/dist-packages/virttest/qemu_vm.py", line 1839, in make_create_command
[stdlog]     devices = qcontainer.DevContainer(
[stdlog]               ^^^^^^^^^^^^^^^^^^^^^^^^
[stdlog]   File "/usr/local/lib/python3.12/dist-packages/virttest/qemu_devices/qcontainer.py", line 220, in __init__
[stdlog]     self._probe_capabilities()
[stdlog]   File "/usr/local/lib/python3.12/dist-packages/virttest/qemu_devices/qcontainer.py", line 453, in _probe_capabilities
[stdlog]     if self.has_qmp_cmd("migrate-set-parameters") and self.has_hmp_cmd(
[stdlog]        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[stdlog]   File "/usr/local/lib/python3.12/dist-packages/virttest/qemu_devices/qcontainer.py", line 805, in has_qmp_cmd
[stdlog]     return cmd in self.__qmp_cmds
[stdlog]            ^^^^^^^^^^^^^^^^^^^^^^
[stdlog] TypeError: argument of type 'NoneType' is not iterable
[stdlog]
[stdlog] Traceback (most recent call last):
[stdlog]   File "/root/elves/data/avocado-vt/virttest/test-providers.d/downloads/io-github-autotest-qemu/qemu/tests/amd_cvm_boot.py", line 75, in run
[stdlog]     vm.create()
[stdlog]   File "/usr/local/lib/python3.12/dist-packages/virttest/error_context.py", line 141, in new_fn
[stdlog]     return fn(*args, **kwargs)
[stdlog]            ^^^^^^^^^^^^^^^^^^^
[stdlog]   File "/usr/local/lib/python3.12/dist-packages/virttest/qemu_vm.py", line 3787, in create
[stdlog]     raise virt_vm.VMStartError(
[stdlog] virttest.virt_vm.VMStartError: VM 'avocado-vt-vm1' failed to start: Error occurred while executing make_create_command(). Check the log for traceback.
..

After fix:

[stdlog] 2026-03-25 06:02:42,188 avocado.virttest.qemu_devices.qcontainer qcontainer       L0179 WARNI| Could not list QMP commands for /root/amdsos_qemu/bin/qemu-system-x86_64 (query-commands line missing or unparsable). Check that this binary runs with '-qmp stdio -vnc none -S' and prints JSON monitor replies on stdout. Migration-related capability probes will assume QMP commands are absent.

[gemini-code-assist]

Code Review

This pull request enhances the QMP command discovery process in virttest/qemu_devices/qcontainer.py. It introduces explicit logging when QMP commands are unparsable or missing and adds a safety check in has_qmp_cmd to handle empty command lists. I have no feedback to provide.

[bssrikanth]

@smitterl @richtja @nickzhq @YongxueHong @zhencliu @cliping @nanli1 @Yingshun @dzhengfy @chloerh @PaulYuuu requesting maintainers review on this pull request.
Thank you for your time in advance.