small qemu improvements

[sharder996]

Small cleanup:

• Moved common stub monitor to test_qemu_backend constructor
• Some tests were using a mock object but were not mocking function calls, so replaced with stub object
• Removed a couple instance variables from the qemu virtual machine class as they are simply copies of members from the already saved VirtualMachineDescription

[ricab]

LGTM, thanks!

Feel free to merge if/when it passes CI.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.85%. Comparing base (5f4dfe2) to head (daddd3d).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/platform/backends/qemu/qemu_virtual_machine.cpp	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3681      +/-   ##
==========================================
- Coverage   88.85%   88.85%   -0.01%     
==========================================
  Files         254      254              
  Lines       14269    14267       -2     
==========================================
- Hits        12679    12677       -2     
  Misses       1590     1590              

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