<fix>[storage]: repair 4.4.52-sm baseline failures

[ZStack-Robot]

Summary

• Guard image snapshot loading when an unattached volume has no VM UUID.
• Serialize backup storage reconnect accounting and cancel reconnect promptly at the threshold.
• Fix the core-local VmPmuConfigCase helper argument shadowing.

Cross-repository CI

• Paired branch: fix/ZSTAC-87030@@2
• Premium companion MR: http://dev.zstack.io:9080/zstackio/premium/-/merge_requests/15062
• Both repositories target 4.4.52-sm.
• This MR replaces zstack!10671 without changing its commit SHA.

Verification

• VmPmuConfigCase stability: PASS.
• BinBuild and UTBuild: PASS in the previous unpaired round.
• BackupStorage reconnect and image snapshot failures are covered by the external UnitTest run.
• The previous UnitTest red cases were Premium baseline failures plus the intentional core_MustFailCase; none maps to this MR's three diffs.
• New paired @@2 CI: pending.

Related: ZSTAC-87030 / premium!15062

sync from gitlab !10723

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📥 Commits

Reviewing files that changed from the base of the PR and between d4f17df and 76be837.

📒 Files selected for processing (3)

• image/src/main/java/org/zstack/image/ImageManagerImpl.java
• storage/src/main/java/org/zstack/storage/backup/BackupStoragePingTracker.java
• test/src/test/groovy/org/zstack/test/integration/kvm/vm/VmPmuConfigCase.groovy

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