labgrid/qemudriver: tie QEMU and QMP monitor start/stop to on_activate()/on_deactivate()

[Bastian-Krause]

Description Until now the QEMU process and QMP monitor start was tied to the on()/off() methods. This feels unnatural, preventing the user from interacting with the QEMU process via monitor commands before the emulation starts and meant starting a new process on each power cycle.

Rework the driver to start QEMU and the QMP monitor in on_activate(), allowing interaction via monitor_command() after activation. The on() and off() methods interact only via QMP now.

All methods relying on a started QEMU and QMP monitor instance are decorated with @Driver.check_active now.

The atexit handling is no longer required since the target's atexit handler already calls the driver's on_deactivate().

While at it, restructure the QEMU mock fixtures in the QEMUDriver tests.

Checklist

• [x] PR has been tested

Alternative to one aspect of #1753

[codecov[bot]]

Codecov Report

:x: Patch coverage is 79.06977% with 9 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 45.2%. Comparing base (70f6150) to head (b9b4489). :white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/driver/qemudriver.py	79.0%	9 Missing :warning:

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1771   +/-   ##
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  Coverage    45.2%   45.2%           
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  Files         172     172           
  Lines       13569   13570    +1     
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+ Hits         6136    6140    +4     
+ Misses       7433    7430    -3     

Flag	Coverage Δ
3.10	45.2% <79.0%> (+<0.1%)	:arrow_up:
3.11	45.2% <79.0%> (+<0.1%)	:arrow_up:
3.12	45.2% <79.0%> (+<0.1%)	:arrow_up:
3.13	45.2% <79.0%> (+<0.1%)	:arrow_up:
3.14	45.2% <79.0%> (+<0.1%)	:arrow_up:
3.9	45.2% <79.0%> (+<0.1%)	:arrow_up:

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[JSydll]

Thanks for addressing this, @Bastian-Krause ! Seems indeed more adherent to the common design/usage patterns. I'll give it a closer look as soon as time permits.

[Bastian-Krause]

Rebased on latest master.