fix: correct VM snapshot usage attribution (two defects)

[waterWang]

Fixes #13921

Two defects corrected in VMSnapshotUsageParser:

1. Wrong snapshot ID label in usage record: createUsageRecord() used the current snapshot's ID (usageRec.getVmSnapshotId()) while duration, size, and disk offering came from the previous event. Changed to previousEvent.getVmSnapshotId().

2. Concurrent snapshot key collision: The unprocessedUsage map keyed entries by vmId + ":" + volId only, so concurrent snapshots of the same volume overwrote each other. Added vmSnapshotId to the key to distinguish them.

Root cause

From the issue body:

Net effect: the first snapshot's usage is credited to the second snapshot, and the first snapshot stops accruing usage even though it still exists and is Ready.

[boring-cyborg]

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[harikrishna-patnala]

thanks for the fix @waterWang, can you please change the branch 4.22 ?

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.65%. Comparing base (d45d481) to head (6cf39e1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../com/cloud/usage/parser/VMSnapshotUsageParser.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #13924      +/-   ##
============================================
- Coverage     19.65%   19.65%   -0.01%     
+ Complexity    19801    19796       -5     
============================================
  Files          6368     6368              
  Lines        574913   574913              
  Branches      70352    70352              
============================================
- Hits         113024   112999      -25     
- Misses       449616   449643      +27     
+ Partials      12273    12271       -2     

Flag	Coverage Δ
uitests	3.41% <ø> (ø)
unittests	20.93% <0.00%> (-0.01%)	⬇️

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