Fix "zpool add -n" for dedup, special and log devices

For dedup, special and log devices "zpool add -n" does not print
correctly their vdev type:

~# zpool add -n pool dedup /tmp/dedup special /tmp/special log /tmp/log
would update 'pool' to the following configuration:
pool

	  /tmp/normal
	  /tmp/dedup
	  /tmp/special
	  /tmp/log

This could lead storage administrators to modify their ZFS pools to
unexpected and unintended vdev configurations.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #9783
Closes #9390