Comment 0 for bug 1826888

In Boostack a customer asked to move ephemeral disk to ceph.

In order to do that we set libvirt-image-backend to rbd with:

```
juju config nova-compute libvirt-image-backend=rbd
```

After the config change was applied any action that involves recreating the vm xml (stop and restart, resize, migrate) are failing for existing vms because we hit the following exception:

https://paste.ubuntu.com/p/vNCCfkTVJz/

In order to fix that we need to revert back the libvirt-image-backend to qcow2 perform the operation needed and then set it back to rbd.

We believe that at least this should be documented in the config.yaml