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

Beginne mit plugins/hypervisors/kvm/src/main/java/org/apache/cloudstack/utils/qemu/QemuCommand.java, insbesondere mit der bestehenden freeze-Implementierung. Verfolge, wie guest-agent-Befehle über die API und UI bereitgestellt werden, definiere dann den unterstützten Befehlsumfang und verifiziere, dass ein Gast-Befehl über beide Schnittstellen gesendet werden kann.

Vom Indexierungsmodell aus dem Issue-Text verfasst.