Ignore exit_io_done= option if no I/O threads are configured

Ignore exit_io_done= option if no I/O threads are configured

The cpuio engine option exit_io_done= should be effective only if
there is at least one real I/O thread configured.

The existing fio_cpuio_queue() with exit_io_done= option enabled
lets cpuio threads exit even if cpuio threads are the only threads
configured.

Since this option is supposed to mean "exit when I/O threads are done",
cpuio threads exiting (but only after the first spin cycle is done)
when no threads have done any I/O is a bit confusing.

> [cpu] exit_on_io_done=bool Detect when IO threads are done, then exit.

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>