Improvement: Filesystem permission check

##### ISSUE TYPE
* Improvement Request

##### COMPONENT NAME

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cloudstack-sysvmadm
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##### CLOUDSTACK VERSION
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N/A
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##### CONFIGURATION

N/A

##### OS / ENVIRONMENT
Cloudstack running on linux systems

##### SUMMARY
Check filesystem permission before upgrading systemvms

##### STEPS TO REPRODUCE

Currently, when issuing the cloudstack-sysvmadm script for upgrading the systemvms (or doing it manually), the process starts without any prechecks.
When there is somehow an issue with filesystem permissions, the new systemvm.iso cannot be copied to the new location. It would be a nice enhancement, when cloudstack had a health check or at least the cloudstack-sysvmadm script a permission check.

A newly introduced health check could continuously check some relevant cloudstack metrics, including the filesystem permissions and show a simple go/nogo somewhere in the admin ui. Or the simple way: a script or the modified sysvmadm script which checks first all storage paths for the correct rights.

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##### EXPECTED RESULTS

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##### ACTUAL RESULTS

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首先定位入口点 cloudstack-sysvmadm，以及它在升级 System VM 之前如何发现存储路径。检查现有的升级流程，并确定预期的权限检查和失败行为。当选定的脚本或 health check 在 systemvm.iso 复制或升级开始之前检测到不可用的权限时，即表示完成。

由索引模型根据 Issue 内容生成。