If PermitRootLogin is disabled live migrations silently fail

[Giuseppe Petralia]

If PermitRootLogin is disabled on compute nodes, live migrations fail but VM remains active running on the source host, making impossible for the operator to understand what is happening.

Error is:

```
2023-10-17 08:34:42.602 3904 ERROR nova.virt.libvirt.driver [-] [instance: 67ba8b07-22b7-4d4b-bf47-2a15d3f4bc45] Live Migration failure: Cannot recv data: Authorized uses only. All activity may be monitored and reported.
root@10.10.10.23: Permission denied (publickey).: Connection reset by peer: libvirt.libvirtError: Cannot recv data: Authorized uses only. All activity may be monitored and reported.
2023-10-17 08:34:42.815 3904 WARNING nova.compute.manager [req-bf622559-f199-4a2d-948f-3962bf686c4d 56b7446f04fa446ca90b1599c729bbe6 c86bb3408a9743f3989b76007787e089 - 4addaf4e7f7d4915bf2387f12bd2b42b 4addaf4e7f7d4915bf2387f12bd2b42b] [instance: 67ba8b07-22b7-4d4b-bf47-2a15d3f4bc45] Received unexpected event network-vif-plugged-b2d4198a-54c2-478d-aa85-39453475a78e for instance with vm_state active and task_state migrating.
2023-10-17 08:34:43.014 3904 ERROR nova.virt.libvirt.driver [-] [instance: 67ba8b07-22b7-4d4b-bf47-2a15d3f4bc45] Migration operation has aborted
2023-10-17 08:34:43.034 3904 ERROR nova.compute.manager [-] [instance: 67ba8b07-22b7-4d4b-bf47-2a15d3f4bc45] Did not find resource allocations for migration 5413f94a-aaac-4359-82b5-81976c0d1c0d on source node compute-9.domain. Unable to revert source node allocations back to the instance.
```

VM should go in error state with an indication about why it failed.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.