[Qemu-block] bug introduced by "block: Move throttling fields from BDS to BB"

[Paolo Bonzini]

Here is next_throttle_token:

-    ThrottleGroup *tg = container_of(blk_bs(blk)->throttle_state,
-                                     ThrottleGroup, ts);
+    BlockBackendPublic *blkp = blk_get_public(blk);
+    ThrottleGroup *tg = container_of(blkp->throttle_state, ThrottleGroup, ts);
     BlockBackend *token, *start;
 
     start = token = tg->tokens[is_write];
 
     /* get next bs round in round robin style */
     token = throttle_group_next_blk(token);
-    while (token != start && !blk_bs(token)->pending_reqs[is_write]) {
+    while (token != start && !blkp->pending_reqs[is_write]) {
         token = throttle_group_next_blk(token);
     }


blkp isn't updated every time token is updated.

The same bug occurs in schedule_next_request.  This patch was only
in 2.7.x.

Thanks,

Paolo