block: replace IOV_MAX with BlockLimits.max_iov

Request merging must not result in a huge request that exceeds the
maximum number of iovec elements.  Use BlockLimits.max_iov instead of
hardcoding IOV_MAX.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/io.c

@@ -1889,7 +1889,8 @@ static int multiwrite_merge(BlockDriverState *bs, BlockRequest *reqs,
             merge = 1;
         }
 
-        if (reqs[outidx].qiov->niov + reqs[i].qiov->niov + 1 > IOV_MAX) {
+        if (reqs[outidx].qiov->niov + reqs[i].qiov->niov + 1 >
+            bs->bl.max_iov) {
             merge = 0;
         }
 

hw/block/virtio-blk.c

@@ -414,7 +414,7 @@ void virtio_blk_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb)
              * 3. merge would exceed maximum transfer length of backend device
              */
             if (sector_num + nb_sectors != req->sector_num ||
-                niov > IOV_MAX - req->qiov.niov ||
+                niov > blk_get_max_iov(blk) - req->qiov.niov ||
                 req->qiov.size / BDRV_SECTOR_SIZE > max_xfer_len ||
                 nb_sectors > max_xfer_len - req->qiov.size / BDRV_SECTOR_SIZE) {
                 submit_requests(blk, mrb, start, num_reqs, niov);