uefi: Do not copy if not necessary

[xaionaro]

Removed extra copies. It reduces memory consumption, improves performance and simplifies the result (it's now flat).

See the justification discussion: #304

[codecov-io]

Codecov Report

Merging #305 into master will decrease coverage by 0.05%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master    #305      +/-   ##
=========================================
- Coverage   39.26%   39.2%   -0.06%     
=========================================
  Files          47      47              
  Lines        3494    3489       -5     
=========================================
- Hits         1372    1368       -4     
+ Misses       1908    1907       -1     
  Partials      214     214

Impacted Files	Coverage Δ
pkg/uefi/biosregion.go	6.66% <0%> (+0.1%)	⬆️
pkg/uefi/firmwarevolume.go	37.11% <100%> (-1.28%)	⬇️
pkg/uefi/file.go	16.55% <100%> (-1.14%)	⬇️

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[GanShun]

Let's discuss this a bit more on the issue. I ran into corruption last time without the copies, but we can do it better.

[xaionaro]

See #304 -- there's more details that are required to be considered, so the patch will be more complicated.

[xaionaro]

@GanShun : A service I implemented based on fiano, consumes too much memory (due to this copies). What you think if we will add support of a special read-only mode (which is required to enable explicitly through a global variable)?

[xaionaro]

Closing the PR in favor to: #326