zpool_get_vdev_prop_value: show missing vdev userprops

If a vdev userprop is not found, present it as value '-', default
source, so it matches the output from pool userprops.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16887