dissect: add commands for attaching/detaching loopback devices

dissect: add commands for attaching/detaching loopback devices

Sometimes it is useful attaching DDIs without mounting them. We could
use "losetup" for that, but doing this in systemd-dissect has various
benefits:

1. we superficially validate the DDI first
2. we set the sector size depending on what we determine
3. we synchronously create the per-partition block devices