chore: remove KVM/VM host (pods) support

[r00ta]

KVM/VM hosts (pods), VM clusters, and machine composition are removed from MAAS. The related REST and websocket endpoints are kept as stubs returning HTTP 410 Gone so that existing API clients get a clear signal instead of a 404, and the install_kvm/register_vmhost deploy parameters are rejected with 400 Bad Request.

Existing LXD and virsh virtual machines remain power-controllable: the lxd and virsh drivers are reintroduced as plain chassis-capable power drivers, so VMs registered before the upgrade keep working as regular machines.

Database changes:

• 0036 drops install_kvm/register_vmhost from maasserver_node
• 0037 drops the pod-only maasserver_bmc columns, the virtualmachine*, podhints, podstoragepool and vmcluster tables, the maasserver_podhost view, and the pod/vmcluster notification triggers
• 0038 normalises leftover bmc_type=POD rows to plain BMCs, preserving the power configuration of existing VMs

Also removed: pod forms and RPC operations, KVM vendor-data generation for machine deployment, pod/vmcluster websocket handlers, VM host, Prometheus metrics and stats, and the corresponding documentation.

[Copilot]

Copilot wasn't able to review this pull request because it exceeds the maximum number of lines (20,000). Try reducing the number of changed lines and requesting a review from Copilot again.

[r00ta]

QA already performed:

• install 3.8/stable
• register a kvm host
• compose 2 vms
• upgrade to this patch
• the machines are still visible and properly working (they can be commissioned/deployed and so on)

General tests with the UI canonical/maas-ui#6114

[skatsaounis]

LGTM 🚀

[skatsaounis]

Still LGTM after the rebase

[tmerten]

🎉