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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