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

从 plugins/hypervisors/kvm/src/main/java/org/apache/cloudstack/utils/qemu/QemuCommand.java 开始，重点查看现有的 freeze 实现。跟踪 guest-agent 命令如何通过 API 和 UI 暴露，然后定义受支持的命令范围，并验证可以通过这两个接口发送 guest 命令。

由索引模型根据 Issue 内容生成。