Dashes for zero latency values in zpool iostat -p

Dashes for zero latency values in zpool iostat -p

This prints dashes instead of zeros for zero latency values in
'zpool iostat -p'.  You'll get zero latencies reported when the
disk is idle, but technically a zero latency is invalid, since you
can't measure the latency of doing nothing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #6210