Virtual Machines Automation from Hypervisors using Qemu-Agent on KVM

##### ISSUE TYPE
* Feature Idea

##### COMPONENT NAME
Categorize the issue, API, KVM, HYP, UI

##### CLOUDSTACK VERSION
Coming versions

##### SUMMARY
Many of us are using Cloudstack to provide public and private cloud services for our internal teams and clients, so normally we don't handle passwords or ssh key password to access the vm's operating system to orchestrate applications lifecycle. So my proposal is that just like user-data works for the first boot we can register actions or commands that we can apply to client's vm using qemu-agent capabilities in KVM so we leverage services like:

- DBaaS
- Managed Applications
- Improvements on CSK

##### STEPS TO REPRODUCE

Cloudstack access the hyp where the vm resides and applies commands like:

virsh qemu-agent-command "$VM_NAME" "$COMMAND_JSON"

首先梳理 CloudStack 如何到达 KVM 虚拟机监控程序，以及 API、UI 和虚拟机监控程序操作的定义位置。阅读 qemu-agent-command 示例，并明确受支持的命令生命周期、权限和失败处理。完成后应包括一个已达成共识的功能范围，以及将 QEMU-agent 操作应用于客户机虚拟机的实现路径。

由索引模型根据 Issue 内容生成。