feat: Support LXD virtual machines as a workshop backend

Workshop currently creates development environments exclusively as LXD containers. This issue proposes adding support for LXD virtual machines (VMs) as an alternative backend, enabling use cases that require stronger isolation, a full kernel, or hardware-level features not available in containers.

Motivation

Some workflows require capabilities that containers cannot provide:

- Kernel module development or custom kernel testing
- Nested virtualisation (e.g. running KVM inside the workshop)
- Stronger isolation guarantees (e.g. security-sensitive workloads)
- Testing OS-level behaviour that differs between container and VM environments

Current behavior

The instance type is hardcoded to [api.InstanceTypeContainer](vscode-file://vscode-app/usr/share/code/resources/app/out/vs/code/electron-browser/workbench/workbench.html) in multiple places across the LXD backend. There is no way for users to request a VM-backed workshop.

Proposed change

Introduce a type field in [workshop.yaml](vscode-file://vscode-app/usr/share/code/resources/app/out/vs/code/electron-browser/workbench/workbench.html) (defaulting to container) that users can set to vm:

```
name: dev
base: ubuntu@24.04
type: vm
limits:
cpu: 4
memory: 4GiB
```