Add Option to Destroy Unreferenced Disks Owned by Guest

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---

#### Description

This feature request proposes adding support for the **"Destroy unreferenced disks owned by guest"** option when destroying a VM, as shown in the Proxmox UI confirmation dialog. This allows for automatic cleanup of orphaned or detached disk volumes that belong to the guest VM but are no longer referenced in the config.

The referenced setting appears in the GUI destroy confirmation dialog and affects only disks no longer referenced in the VM configuration. This option is especially useful for managing storage resources efficiently in environments where stale or orphaned disks might persist after broken build pipelines or testing workflows.

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#### Use Case(s)

- Automatically clean up unreferenced guest-owned disks when destroying VMs, reducing manual cleanup effort.
- Prevent accumulation of unused storage volumes that are no longer attached to any VM.
- Improve automation in CI pipelines, ephemeral VM lifecycles, and temporary environments.
- Support parity with Proxmox GUI functionality when using API- or CLI-based workflows.

---

#### Potential Configuration

#### Packer example:

```hcl
"builders": [
{
"type": "proxmox-iso",
"destroy_unreferenced_disks": true,
# ...
}
```

#### CLI example:

```bash
qm destroy 117 --destroy-unreferenced-disks 1
```

---

### Potential References

- Proxmox GUI VM destroy confirmation dialog includes a checkbox:
> [✓] Destroy unreferenced disks owned by guest
> Referenced disks will always be destroyed.

- [`qm destroy` documentation](https://pve.proxmox.com/pve-docs/qm.1.html)
```