qemu crash for  suspend and resume vm while backup disk of vm
Description of problem:
![image](/uploads/40e41df2dab7e0d3dacb6c07c1bf42b1/image.png)
Steps to reproduce:
1. virsh create vm2.xml
2. virsh backup-begin domid
3. virsh suspend domid
4. sleep 1 && virsh resume domid

qemu crash
Additional information:
static int blk_do_set_aio_context(BlockBackend *blk, AioContext *new_context,
                                  bool update_root_node, Error **errp)
{
    BlockDriverState *bs = blk_bs(blk);
    ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
    int ret;

    if (bs) {
        bdrv_ref(bs);

        if (update_root_node) {
            ret = bdrv_child_try_set_aio_context(bs, new_context, blk->root,
                                                 errp);
            if (ret < 0) {
                bdrv_unref(bs);
                return ret;
            }
        }
        if (tgm->throttle_state) {
         _   ****bdrv_drained_begin(bs);----- bs->aio_context->lock lock count is 0,so unlock failed**_
            throttle_group_detach_aio_context(tgm);
            throttle_group_attach_aio_context(tgm, new_context);
            bdrv_drained_end(bs);
        }

        bdrv_unref(bs);
    }

    blk->ctx = new_context;
    return 0;
}