Bug: volumes for disposable vms never get destroyed

Using a zpool as driver for vm storage, I notice that disposables leave behind their datasets:

```
$ qvm-ls | grep 'disp[0-9]' | wc -l
0

$ zfs list | grep 'disp[0-9]' | wc -l
9001
```

These have to be manually destroyed:

```
$ zfs list | grep -o 'disp[0-9][0-9]*' | grep -Ev 'EXCEPT|THESE' | xargs -I{} sudo zfs destroy {}
```
