kboot: Add support for ZFS volumes

Add the zfs device and filesystem to config and write the hook we need
to probe zfs since there's not a generic mechanism in place to do that
when ZFS is configured.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38008

(cherry picked from commit f20ecce33a2a9a992fb267edf02edaa6eb9178b3)