server: treat a zero-capacity storage pool as full

[nagaboinaramgopal]

checkUsagedSpace divided used by total capacity without guarding a zero total. A pool reporting capacityBytes == 0 with zero used produced NaN, and NaN >= threshold is false, so the method returned true and reported that a pool with no capacity had space, letting allocation or resize proceed onto it. Return false when the pool reports no capacity.

Tested: new unit test checkUsagedSpaceReturnsFalseForZeroCapacityPool (fails before, passes after); StorageManagerImplTest green.

[DaanHoogland]

it is a long shot but a negative value may have been entered in the DB and coming back through a VO, so clgtm.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 19.78%. Comparing base (b7a834b) to head (7114e88).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ain/java/com/cloud/storage/StorageManagerImpl.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #14052      +/-   ##
============================================
- Coverage     19.78%   19.78%   -0.01%     
+ Complexity    19998    19996       -2     
============================================
  Files          6371     6371              
  Lines        575899   575901       +2     
  Branches      70495    70496       +1     
============================================
- Hits         113967   113959       -8     
- Misses       449501   449512      +11     
+ Partials      12431    12430       -1     

Flag	Coverage Δ
uitests	3.53% <ø> (ø)
unittests	21.06% <50.00%> (-0.01%)	⬇️

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