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

Start with plugins/hypervisors/kvm/src/main/java/org/apache/cloudstack/utils/qemu/QemuCommand.java, especially the existing freeze implementation. Trace how guest-agent commands are exposed through the API and UI, then define the supported command scope and verify that a guest command can be sent through both interfaces.

Written by the indexing model from the issue text.