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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先找出進入點 cloudstack-sysvmadm，以及它在升級 System VM 之前如何探索儲存路徑。檢視現有的升級流程，並確認預期的權限檢查與失敗行為。當選定的 script 或 health check 能在 systemvm.iso 複製或升級開始之前偵測到無法使用的權限時，即表示完成。

由索引模型根據 Issue 內容生成。