The problem is that nvme block devs on Linux have a radically different
naming convention (nvmeXnYpZ).  So, I just supported that.  I was going
to try to understand all the naming conventions, but I couldn't quickly
find a document that claimed to describe them all.  Oh well.

(I also added a fallback autodetect the rootdev on Linux that uses
newer methodologies, back when I hadn't understood the problem.)