fio2gnuplot: Don't truncate fio log files

fio2gnuplot: Don't truncate fio log files

The code was stopping at reading once the shorter stopped. But in the
case of a very big difference of performances between hosts, some could
be much longer/shorter than others.

This patch does insure that until _all_ input files got read complety we
do had fake data with a negative time.

This way, all the traces will be complete as per fio's output.