Zero size recurring snapshots piling up after server is shutdown

Snapshot taking process should follow the following logic:

1. If there are zero snapshots but there is a recurring schedule set, then it should still take the first snapshot for completion sake (regardless of whether Volume is detached or attached to a VM which is running or stopped).
2. If there are existing snapshots, and the VM was up after the last snapshot, then take a snapshot (regardless of whether the VM is currently up or down or volume is detached since then)
3. If there are existing snapshots, but the VM is currently down, AND the last snapshot was after the VM was shutdown, Skip the snapshot and then ADD EVENT - SKIPPING Snapshot for Volume ROOT-X/DATA-X indicating that it is skipped because VM is not running - this Event will help reduce future confusion when someone starts the server and thinks that bunch of days of snapshots are missing.

##### ISSUE TYPE

Bug Report

##### COMPONENT NAME

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##### CLOUDSTACK VERSION

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4.17
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##### CONFIGURATION

N/A

##### OS / ENVIRONMENT

N/A

##### SUMMARY

User has recurring snapshots on a Volume with KEEP 5 setting.
There are snapshots present already and 5 are seen in UI.
User shuts down the VM and leaves it off for several days.
Each day, there is a ZERO size snapshot entry made, but in backend it points to the same last snapshot (checked via SQL).
But on Display there are tens of snapshots present and this counts against the MAX 100 snapshots per account (I know I can increase this to higher number).
Users are confused why so many snapshots.

NOTE - as a technical person, I understand why having the Zero size snapshot entry makes sense - essentially shows that the process ran as expected. However from user perspective, it is confusing especially when the server is down for weeks, and they come back to Max snapshots exceeded errors.

##### STEPS TO REPRODUCE

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User has recurring snapshots on a Volume with KEEP 5 setting.
There are snapshots present already and 5 are seen in UI.
User shuts down the VM and leaves it off for several days.
Each day, there is a ZERO size snapshot entry made, but in backend it points to the same last snapshot (checked via SQL).
But on Display there are tens of snapshots present and this counts against the MAX 100 snapshots per account (I know I can increase this to higher number).
Users are confused why so many snapshots.
~~~

##### EXPECTED RESULTS

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1. Zero size snapshots should not show up in the UI, and should not form a dependency in the cloud database's snapshot table.
OR Perhaps indicate zero size snapshot in the UI so users know without contacting Admins.

4. The zero size snapshots should not count against the Max 100 limit.
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##### ACTUAL RESULTS

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zero size snapshots count against max snapshots.
large number of snapshots in UI confuses the users
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首先使用一台已停止的 VM 重现定期 snapshot 过程，然后检查 UI 和云数据库的 snapshot 表，查找所述的零大小条目。比较它们的可见性以及它们对帐户最大 snapshot 限制的贡献，并验证所选的预期行为：抑制、识别或排除这些条目。

由索引模型根据 Issue 内容生成。