genfio: Adding iodepth support

It's useful to be able to specify the iodepth while running fio with
libaio.

By default, we keep iodepth=1 to keep the same as fio's default.
genfio: Enforce time_based if runtime > 0

If the device is short and the bandwidth pretty high, we can exit
_before_ runtime is achieved. To avoid this phenomena, let's use
time_based directive that will ensure that no run will be shorten.