Turn the CPU burner into a real io engine

This removes the special casing in fio.c for the cpu engine, and
also gets rid of FIO_CPUIO.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/log.c b/log.c
index 994f497..50caf3d 100644
--- a/log.c
+++ b/log.c
@@ -172,7 +172,7 @@
 {
 	int ret = 0;
 
-	if (td->io_ops->flags & FIO_CPUIO)
+	if (td->io_ops->flags & FIO_DISKLESSIO)
 		return 0;
 
 	if (td->read_iolog_file)