Improve output of qvm-vm-update for not-to-update vms

[techge]

The first commit might be sufficient to fix the issue at hand, but the second makes it more clear.

Please let me know if I missed some common code quality details. First commit in a while...

Closes QubesOS/qubes-issues#9749

[marmarek]

See CI logs, it wants black -l80

[marmarek]

Other than that, looks okay

[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.42%. Comparing base (32a14bb) to head (2d1eaf6).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
vmupdate/vmupdate.py	60.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
- Coverage   71.95%   71.42%   -0.53%     
==========================================
  Files          12       12              
  Lines        1330     1344      +14     
==========================================
+ Hits          957      960       +3     
- Misses        373      384      +11     

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