stand/zfs: Move spa_find_by_dev from zfsimpl.c to zfs.c

zfsimpl.c doesn't know about devdesc at all, but zfs.c does. Move it to
zfs.c, which is the only user. Keep it static for now, but it could be
exposed later if something else were to need it.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D39408