stand: Move i386_devdesc to a union

Rather than have the magic, hand-crafted fields that have to align with
fields in other structures at the end of i386_devdesc, make it into
anonymous union and adjust the code accordingly. This is safer and
similar to what CAM does.

Sponsored by: Netflix
Reviewed by: kevans, tsoome (prior version)
Differential Revision: https://reviews.freebsd.org/D35965

(cherry picked from commit f197c0bf3e075286ccea32cd12023f3317474c5a)