uefi: Add a log when not parsing FV content

[JulienVdG]

This would have helped to understand #289 without opening the code

[codecov-io]

Codecov Report

Merging #290 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
- Coverage   39.26%   39.25%   -0.02%     
==========================================
  Files          47       47              
  Lines        3433     3434       +1     
==========================================
  Hits         1348     1348              
- Misses       1871     1872       +1     
  Partials      214      214

Impacted Files	Coverage Δ
pkg/uefi/firmwarevolume.go	40% <0%> (-0.45%)	⬇️

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