fio2gnuplot: Keep original filename in temp. files

It was pretty hard to understand what trace file was linked to which fio
output file. This simple commit add a simpler header to inform the
debugger which fio outputfile got used.
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.
fio2gnuplot: Don't plot fake data

As we did add some fake data to keep as long a possible the coherency
between all the fio files, let's ignore them when rendering.

As a consequence, we are keeping the time associated to each
performance we store instead of a single time for all performance
traces. This will increase precision while plotting.