Add vsync io engine

It uses readv/writev to transfer the data and coalesces adjacent
data into a single system call (emulating queueing).

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/options.c b/options.c
index 3de2ae6..1e99810 100644
--- a/options.c
+++ b/options.c
@@ -440,6 +440,9 @@
 			  { .ival = "psync",
 			    .help = "Use pread/pwrite",
 			  },
+			  { .ival = "vsync",
+			     .help = "Use readv/writev",
+			  },
 #ifdef FIO_HAVE_LIBAIO
 			  { .ival = "libaio",
 			    .help = "Linux native asynchronous IO",