stand: For zfs, set dv_fmtdev to zfs_fmtdev

Add a generic way to get the string representation of a zfs device / mount.

Sponsored by: Netflix
Reviewed by: tsoome (prior version)
Differential Revision: https://reviews.freebsd.org/D35923

[ Dropped changes to stand/libsa/zfs/Makefile.inc file, depends

on newer OpenZFS ]

(cherry picked from commit 654b7837a6a08ec5b51635badd14794754fab7c5)