[sidshas03]

Description

This PR fixes a data loss problem that shows up when a volume snapshot of a
running VM gets interrupted midway, either because the agent dies or the merge
fails silently. In that situation the VM keeps running off an overlay file,
but volumes.path is still pointing at the base. So the next time the VM
starts, it comes up on the base and all the writes that went to the overlay
are gone, with nothing telling you it happened.

There are a few parts to the fix. Script.runSimpleBashScriptWithFullResult()
now carries the actual virsh error output in the exception it throws. Before
this the retry logic had nothing to match against, so now it can recognise the
transient ones like "disk already in active block job" and retry properly. The
snapshot markers also store vmName, diskLabel and overlayPath now, which gives
reconcileSnapshotMarkers() enough to work with, and that method is implemented
fully in this PR. For a running VM that is sitting on an overlay it does a
blockcommit, and for the offline case it falls back to qemu-img commit, with
safety checks so a modified base file is never overwritten.

Along with that, waitForCommandsAndExit is now waitForCommandsAndReconnect. It
calls reconnect() instead of System.exit(1), because exiting was killing
snapshot merges that were still in flight. And there was a duplicate
SNAPSHOT_MARKER_DIR constant that was breaking the build, so that is removed.

Fixes: #14691

Types of changes

• Bug fix (non-breaking change which fixes an issue)

[boring-cyborg]

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[codecov]

Codecov Report

❌ Patch coverage is 0.46948% with 212 lines in your changes missing coverage. Please review.
✅ Project coverage is 20.76%. Comparing base (2f34b0c) to head (2b120bd).

Files with missing lines	Patch %	Lines
...ud/hypervisor/kvm/storage/KVMStorageProcessor.java	0.54%	184 Missing ⚠️
agent/src/main/java/com/cloud/agent/Agent.java	0.00%	23 Missing ⚠️
...s/src/main/java/com/cloud/utils/script/Script.java	0.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #14697      +/-   ##
============================================
- Coverage     20.77%   20.76%   -0.01%     
+ Complexity    20847    20846       -1     
============================================
  Files          6053     6053              
  Lines        548222   548428     +206     
  Branches      73236    73268      +32     
============================================
- Hits         113896   113895       -1     
- Misses       420984   421191     +207     
  Partials      13342    13342              

Flag	Coverage Δ
uitests	3.86% <ø> (ø)
unittests	22.20% <0.46%> (-0.01%)	⬇️

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