zpool: Dryrun fails to list some devices

zpool create -n fails to list cache and spare vdevs.
zpool add -n fails to list spare devices.
zpool split -n fails to list special and dedup labels.
zpool add -n and zpool split -n shouldn't list hole devices.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #11122
Closes #11167