Add API/UI option to send guest command via qemu agent

##### ISSUE TYPE
* Enhancement Request
* Feature Idea

##### COMPONENT NAME
~~~
API/UI/Backend
~~~

##### CLOUDSTACK VERSION
~~~
4.19
~~~

##### CONFIGURATION

##### OS / ENVIRONMENT

##### SUMMARY
Hi team,

I notice that actally, for vm that have qemu agent installed, is not possible through cloudstack send command to the host, like vmware guest agent tool.
This is possible with qemu agent and i notiche that `freeze` for example was implemented with this class:
https://github.com/apache/cloudstack/blob/main/plugins/hypervisors/kvm/src/main/java/org/apache/cloudstack/utils/qemu/QemuCommand.java

But is not implemented for all type of command and I don't see any API that cover it ( and also via UI I can't see any "form").

This is something that you are interested to implemet?

##### STEPS TO REPRODUCE

##### EXPECTED RESULTS

##### ACTUAL RESULTS

plugins/hypervisors/kvm/src/main/java/org/apache/cloudstack/utils/qemu/QemuCommand.java から始め、特に既存の freeze 実装を確認します。guest-agent コマンドが API と UI を通じてどのように公開されているかを追跡し、サポートするコマンドの範囲を定義してから、両方のインターフェースを通じてゲストコマンドを送信できることを検証します。

索引モデルが issue の本文から書いたものです。