tools/qvm-run: translate negative return codes (signals) to positive exit codes

[EliasDevv]

When qvm-run is interrupted via SIGINT (ctrl-c), subprocess.wait()
returns a negative value (-signal). This translates negative return
codes to positive exit codes using abs(retcode) + 128, consistent
with shell signal handling conventions.

Fixes QubesOS/qubes-issues#10468

[EliasDevv]

I'm aware the commits are not GPG-signed yet. I'm setting up commit signing and will force-push signed commits shortly.

[ben-grande]

Please change the title of the commit message. "Translating return code" is what had to be done to accomplish the proposed goal. What this commit is about is exiting with the appropriate code.

[marmarek]

For Ctrl-c, the except section applies, and here similar conversion is needed.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.36%. Comparing base (576fa1c) to head (d770f79).
⚠️ Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
qubesadmin/tools/qvm_run.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #457      +/-   ##
==========================================
+ Coverage   76.35%   76.36%   +0.01%     
==========================================
  Files          53       53              
  Lines        9388     9401      +13     
==========================================
+ Hits         7168     7179      +11     
- Misses       2220     2222       +2     

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[marmarek]

@EliasDevv are you still working on this?