block: Inherit opt_transfer_length

When there is a format driver between the backend, it's not guaranteed
that exposing the opt_transfer_length for the format driver results in
the optimal requests (because of fragmentation etc.), but it can't make
things worse, so let's just do it.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NWenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NBenoît Canet <benoit@irqsave.net>

block.c

@@ -489,7 +489,25 @@ static int bdrv_refresh_limits(BlockDriverState *bs)
 
     memset(&bs->bl, 0, sizeof(bs->bl));
 
-    if (drv && drv->bdrv_refresh_limits) {
+    if (!drv) {
+        return 0;
+    }
+
+    /* Take some limits from the children as a default */
+    if (bs->file) {
+        bdrv_refresh_limits(bs->file);
+        bs->bl.opt_transfer_length = bs->file->bl.opt_transfer_length;
+    }
+
+    if (bs->backing_hd) {
+        bdrv_refresh_limits(bs->backing_hd);
+        bs->bl.opt_transfer_length =
+            MAX(bs->bl.opt_transfer_length,
+                bs->backing_hd->bl.opt_transfer_length);
+    }
+
+    /* Then let the driver override it */
+    if (drv->bdrv_refresh_limits) {
         return drv->bdrv_refresh_limits(bs);
     }