remove instanceof checks and use reflection for admin cmd invocations

##### ISSUE TYPE
* cleanup

##### COMPONENT NAME

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API
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##### CLOUDSTACK VERSION

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4.17
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##### CONFIGURATION

##### OS / ENVIRONMENT

##### SUMMARY

the API classes have instances that are for root admin only, these are checked for using `instanceof` in the service layer. This should be cleaned up in favour for reflection invocations as in #6951. It would be better to do all in one go but the fix in #6951 really needs it and is for a point release, so postponing.

##### STEPS TO REPRODUCE

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##### EXPECTED RESULTS

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##### ACTUAL RESULTS

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從處理 admin 命令呼叫的 API 類別和服務層著手，接著比較 #6951 中描述的 reflection 方法。追蹤針對僅限 root admin 實例的現有 instanceof 檢查，並找出受影響的呼叫路徑。當這些檢查使用預期的 reflection 方法，同時保留僅限 admin 的行為時，即表示完成。

由索引模型根據 Issue 內容生成。