cardiff: Don't count disk's twice in summmary mode

Prior this commit, if systems had more than 1 disk, the counting wasn't
reset leading to a wrong number of disks.

Before:
simultaneous_write_1M_IOps     sda: SUMMARY :  15 unstable   hosts with    41.63 IOps as average value and     9.09 standard deviation
simultaneous_write_1M_IOps     sdb: SUMMARY :  30 unstable   hosts with    41.63 IOps as average value and     9.09 standard deviation

There isn't 30 sdb disks but only 15...

After:
simultaneous_write_1M_IOps     sda: SUMMARY :  15 unstable   hosts with    41.63 IOps as average value and     9.09 standard deviation
simultaneous_write_1M_IOps     sdb: SUMMARY :  15 unstable   hosts with    41.63 IOps as average value and     9.09 standard deviation