genfio: Adding runtime=0 support

If the runtime is set to 0, it will ask fio to get until the end of the
device. This could be useful in some cases.
genfio: Adding cached_based option

By default fio is still run in direct mode which is safer for many
benchmarking approach. In some case, you might need to enable the
cached-based IOs.

The "-c" option is now here for enabling this.