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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Start in the API classes and service layer handling admin command invocations, then compare the reflection approach described in #6951. Trace the existing instanceof checks for root-admin-only instances and identify the affected invocation paths. Done means those checks use the intended reflection approach while preserving the admin-only behavior.

Written by the indexing model from the issue text.