zpool: allow relative vdev paths

zpool create won't let you use relative paths to disks. This is
annoying when you want to do:

zpool create tank ./diskfile

But have to do..

zpool create tank pwd/diskfile

This fixes it.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #17042