Fix UC image size

[upils]

Since the Go rewrite we were adding 34*1024 sectors of 512 bytes at the end of the disk to account for the GPT backup header. This was too much and was hiding the fact that we were not calculating the size needed by the structures defined in the gadget.yaml.

Now that the partition handling has been reworked, some room was missing.

Fix the issue by using the max volume size as the minimum size needed.

Solves LP: #2077572.
Closes: FR-9159

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.91%. Comparing base (c7446c8) to head (5f0bb9e).

Files with missing lines	Patch %	Lines
internal/statemachine/common_states.go	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #251      +/-   ##
==========================================
- Coverage   93.96%   93.91%   -0.05%     
==========================================
  Files          18       18              
  Lines        3429     3437       +8     
==========================================
+ Hits         3222     3228       +6     
- Misses        133      134       +1     
- Partials       74       75       +1     

Flag	Coverage Δ
unittests	93.91% <86.66%> (-0.05%)	⬇️

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[upils]

Waiting for canonical/snapd#14561 to land before cleaning and merging this.