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/HOWTO b/HOWTO
index a20f714..3d6b354 100644
--- a/HOWTO
+++ b/HOWTO
@@ -344,6 +344,8 @@
 
 			psync 	Basic pread(2) or pwrite(2) io.
 
+			vsync	Basic readv(2) or writev(3) IO.
+
 			libaio	Linux native asynchronous io.
 
 			posixaio glibc posix asynchronous io.