kboot: Rework hostdisk.c to allow easier ZFS support.

Keep a list of disks and partitions that we have. Keep track of the
sizes of the media and sector and use that to implement DIOCGMEDIASIZE
and DIOCGSECTORSIZE. Proivde a way to lookup disks by name.

Sponsored by: Netflix
Reviewed by: kevans (prior version)
Differential Revision: https://reviews.freebsd.org/D38013