Don't run 'zpool iostat -c CMD' command on all vdevs, if vdevs specified

Don't run 'zpool iostat -c CMD' command on all vdevs, if vdevs specified

zpool iostat allows you to specify only certain vdevs to display.
Currently, if you run 'zpool iostat -c CMD vdev1 vdev2 ...'
on specific vdevs, it will actually run the command on *all* vdevs,
and just display the results for the vdevs you specify.  This patch
corrects the behavior to only run the command on the specified vdevs,
and also enables the zpool_iostat_005_pos.ksh tests.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #5443