Extend aux label to add path information

Pool import logic uses vdev paths, so it makes sense to add path
information on AUX vdev as well.

Reviewed-by: Umer Saleem <usaleem@ixsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #15737