Dir clean up on delete

I don;t know whether this was intentional, but while before deletes cleared the dir structure back up to the namespace, now we leave the name dir as well.

Close if this is meant to be (it does look weird with dead capmvm names lying around)

Before delete:
```
/var/lib/flintlock/vm///
```

After delete:
```
/var/lib/flintlock/vm//
```

Ideally we would remove back to
```
/var/lib/flintlock/vm/
```
if all identically names mvms in that namespace are removed, and

```
/var/lib/flintlock/vm
```
if all mvms in that namespace are removed

Trace the delete lifecycle responsible for the paths shown in the issue, starting with the code that removes a VM's UID directory. Verify how deletion behaves when the last VM with a name or namespace is removed, and confirm that the namespace and /var/lib/flintlock/vm directories are retained or removed as described.

Written by the indexing model from the issue text.