Feature Idea: 'Host/Cluster Waiting For Maintenance' Mode

##### ISSUE TYPE

* Improvement Request

##### COMPONENT NAME

~~~
Host? Cluster? Im not sure
~~~

##### CLOUDSTACK VERSION

~~~
NA
~~~

##### CONFIGURATION

##### OS / ENVIRONMENT

##### SUMMARY

### **Current Capability**
CloudStack currently offers a 'Maintenance' Mode, which facilitates the live migration of all VMs from a host and removes the host from the cluster for maintenance.

### **Proposed Feature: "Waiting for Maintenance" Mode**
The proposed "Waiting for Maintenance" Mode introduces a preparatory state that addresses scenarios where live migration is impractical or impossible. This feature would enable gradual decommissioning or maintenance while avoiding service disruption.

### **General Idea of How It Might Work:**

**1. **Operator Responsibilities:****
- Customer communication and notification will be managed entirely by the cloud company, outside of CloudStack. This is to inform customers that they are given a time window to voluntarily restart their VMs before the cut off date.

**2. CloudStack Responsibilities:**
- Block the creation of new VMs to the host/cluster marked as 'Waiting For Maintenance'
- Ensure restarted VMs are relocated to clusters with matching host tags.

_This is actually a similar process as how AWS Cloud does it: https://aws.amazon.com/maintenance-help/_

### **Use Cases**
**Scenario 1: Decommissioning an Old Compute Cluster**

Problem:
- Legacy clusters with outdated CPU architectures cannot perform live migration due to compatibility issues
(e.g., VM freezing during migration causing downtime).
- Existing VMs must restart to migrate to a new cluster with compatible architectures.
- The old cluster remains active, risking the placement of new VMs and hindering decommissioning.

**Scenario 2: Maintenance of GPU Clusters with GPU Passthrough**

Problem:
- GPU passthrough prevents live migration, unlike vGPU setups that allow seamless migration.
- Downtime-free maintenance is not feasible, requiring customer cooperation to restart affected VMs.

##### STEPS TO REPRODUCE

~~~
NA
~~~

##### EXPECTED RESULTS

~~~
Refer to Above
~~~

##### ACTUAL RESULTS

~~~
Not able to facilitate smooth decomissioning of servers for compute where live migration is not possible.
~~~

首先阅读 CloudStack 现有的主机和集群维护模式行为，然后追踪 VM 创建、重启、迁移以及基于主机标签的放置是如何处理的。定义操作员可见的状态及其对主机和集群的影响，包括重启后的 VM 如何放置；当新 VM 避开已标记的资源，并且重启后的 VM 在不干扰现有工作负载的情况下到达兼容的带标签集群时，即视为完成。

由索引模型根据 Issue 内容生成。