stand/zfs: Refactor zfs_get_bootenv

Create a new interface to zfs_get_bootenv called zfs_get_bootenv_spa
which takes a spa instead of a void * (effectively a devdesc *). Use
that in zfs_get_bootenv.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D39409