If we go over the maximum number of iovecs support by syscall we get
back EINVAL from the kernel which translate to I/O errors for the guest.

Add a MAX_IOV defintion for platforms that don't have it.  For now we use
the same 1024 define that's used on Linux and various other platforms,
but until the windows block backend implements some kind of vectored I/O
it doesn't matter.

Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: qemu/block.c
===================================================================
--- qemu.orig/block.c	2010-01-26 10:59:39.757004445 +0100
+++ qemu/block.c	2010-01-26 11:01:38.056023231 +0100
@@ -1689,6 +1689,10 @@ static int multiwrite_merge(BlockDriverS
             merge = bs->drv->bdrv_merge_requests(bs, &reqs[outidx], &reqs[i]);
         }
 
+        if (reqs[outidx].qiov->niov + reqs[i].qiov->niov + 1 > IOV_MAX) {
+            merge = 0;
+        }
+
         if (merge) {
             size_t size;
             QEMUIOVector *qiov = qemu_mallocz(sizeof(*qiov));
Index: qemu/qemu-common.h
===================================================================
--- qemu.orig/qemu-common.h	2010-01-26 14:41:40.894254285 +0100
+++ qemu/qemu-common.h	2010-01-26 14:42:27.267275698 +0100
@@ -54,6 +54,10 @@ struct iovec {
     void *iov_base;
     size_t iov_len;
 };
+/*
+ * Use the same value as Linux for now.
+ */
+#define IOV_MAX		1024
 #else
 #include <sys/uio.h>
 #endif