genfio: Adding prefix to fio filename

This patch add a prefix (-x) option to add context to fio jobs.
When running many many jobs it's pretty hard to remember which job was
made for.

Adding a prefix to the filename can give some interesting information
like the tested hardware or the day we run it, or a option you test
etc...

If the prefix owns a / in it, it will be considered as a directory and
will create it. The fio jobs will be stored into this directory.