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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从处理 admin 命令调用的 API 类和服务层入手，然后对比 #6951 中描述的 reflection 方法。跟踪针对仅限 root admin 实例的现有 instanceof 检查，并确定受影响的调用路径。当这些检查使用预期的 reflection 方法，同时保留仅限 admin 的行为时，即表示完成。

由索引模型根据 Issue 内容生成。