Figure out how nested partition tables offsets work

[ralphje]

We don't add the offset of partition tables to their nested volumes (this happens in the base of BSD, where a MBR will contain a BSD volume system).

However, more by luck than by accident, this seems to work fine in this case: the BSD disklabel actually lists all volume offsets from the start of the disk, so we're fine not adding self.offset there.

However, I'm not sure this is the case for all nested volume systems. This requires some research.