Qemu enhancements for extra_args

[a3f]

Description

• what do you use the feature for?

The barebox test suite uses Labgrid in CI for QEMU and these are some quality of life improvements around QEMUDriver extra_args:

• Allow extra_args to be empty

• Show QEMU stderr for malformed extra_args

• Allow extra_args to list display devices and have them shown if requested

• how did you verify the feature works?

Manually by editing the YAML and running Labgrid with QEMUDriver:

• by removing extra_args
• adding an unknown option in extra_args
• adding -device ramfb in extra_args and setting display to qemu-default

Checklist

• Documentation for the feature
• Tests for the feature
• The arguments and description in doc/configuration.rst have been updated
• Add a section on how to use the feature to doc/usage.rst
• Add a section on how to use the feature to doc/development.rst
• [x ] PR has been tested

[Emantor]

Nit in the last commit message: replace last now with not. Otherwise looks good.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 56.2%. Comparing base (587a872) to head (bd60c1c).
Report is 4 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/driver/qemudriver.py	33.3%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1560     +/-   ##
========================================
- Coverage    56.2%   56.2%   -0.1%     
========================================
  Files         170     170             
  Lines       13247   13248      +1     
========================================
  Hits         7449    7449             
- Misses       5798    5799      +1     

Flag	Coverage Δ
3.10	56.2% <33.3%> (-0.1%)	⬇️
3.11	56.2% <33.3%> (-0.1%)	⬇️
3.12	56.2% <33.3%> (-0.1%)	⬇️
3.13	56.1% <33.3%> (-0.1%)	⬇️
3.9	56.2% <33.3%> (-0.1%)	⬇️

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