kboot: Add ZFS support to hostdisk

Add helper function to walk through the disk drives we've found to look
for zpools. main.c will still need to call this because the loader
hasn't implemented a good way to 'taste' drives for zpools and/or GELI
partitions (mostly because there's no generic list of candidate
devices).

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38007