Define OS preferred IO engine

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/options.c b/options.c
index 67d0d2d..de2d037 100644
--- a/options.c
+++ b/options.c
@@ -918,7 +918,7 @@
 		.type	= FIO_OPT_STR_STORE,
 		.off1	= td_var_offset(ioengine),
 		.help	= "IO engine to use",
-		.def	= "sync",
+		.def	= FIO_PREFERRED_ENGINE,
 		.posval	= {
 			  { .ival = "sync",
 			    .help = "Use read/write",