S390 device map

yeah, I think it can be nice method in Storage to have something like `GetPartitionForPath` which tries hard to find on which partition  path lives. This way I do not know clear and easy way how to make it without magic. The nearest is
```
boot_part = ["/boot/zipl", "/boot", "/"].map{|p| Storage.GetEntryForMountpoint(p)}.find{ |p| !p.empty? }
```