Amend MAX_EVENTS to 256

From: Tim Smith <tim.smith@citrix.com>

The MAX_EVENTS value of 1024 causes us to run out of a perfectly reasonable
/proc/sys/fs/aio-max-nr limit when using many disks on a single host.
Reduce this value down to conserve resource.
---
 block/linux-aio.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/linux-aio.c b/block/linux-aio.c
index 4c423fcccf..2cbab594dc 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -26,7 +26,7 @@
  *      than this we will get EAGAIN from io_submit which is communicated to
  *      the guest as an I/O error.
  */
-#define MAX_EVENTS 1024
+#define MAX_EVENTS 256
 
 /* Maximum number of requests in a batch. (default value) */
 #define DEFAULT_MAX_BATCH 32
