dm: virtio-block: extend the max iov number of virtio block

It is found that windows will issue blkio operation with amount of
sectors in one request. This patch extends the max iov number of
virtio block to 256.

Tracked-On: #2962
Signed-off-by: Yu Wang <yu1.wang@intel.com>
Acked-by: Yin Fengwei <fengwei.yin@intel.com>

devicemodel/include/block_if.h

@@ -39,7 +39,7 @@
 #include <sys/uio.h>
 #include <sys/unistd.h>
 
-#define BLOCKIF_IOV_MAX		33	/* not practical to be IOV_MAX */
+#define BLOCKIF_IOV_MAX		256	/* not practical to be IOV_MAX */
 
 struct blockif_req {
 	struct iovec	iov[BLOCKIF_IOV_MAX];