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 如何連接到 KVM 虛擬機監控程式，以及 API、UI 和虛擬機監控程式動作的定義位置。閱讀 qemu-agent-command 範例，並釐清受支援命令的生命週期、權限和失敗處理。完成後應包含一個已達成共識的功能範圍，以及將 QEMU-agent 動作套用至客體虛擬機器的實作路徑。

由索引模型根據 Issue 內容生成。