Revert "block: Forbid I/O throttling on nodes with multiple parents for 2.6"

This reverts commit 76b223200e.

Now that I/O throttling is fully done on the BlockBackend level, there
is no reason any more to block I/O throttling for nodes with multiple
parents as the parents don't influence each other any more.

Conflicts:
	block.c

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -1522,12 +1522,6 @@ static int bdrv_open_inherit(BlockDriverState **pbs, const char *filename,
             return -ENODEV;
         }
 
-        if (bs->blk && blk_get_public(bs->blk)->throttle_state) {
-            error_setg(errp, "Cannot reference an existing block device for "
-                       "which I/O throttling is enabled");
-            return -EINVAL;
-        }
-
         bdrv_ref(bs);
         *pbs = bs;
         return 0;

blockdev.c

@@ -2652,13 +2652,6 @@ void qmp_block_set_io_throttle(const char *device, int64_t bps, int64_t bps_rd,
         goto out;
     }
 
-    /* The BlockBackend must be the only parent */
-    assert(QLIST_FIRST(&bs->parents));
-    if (QLIST_NEXT(QLIST_FIRST(&bs->parents), next_parent)) {
-        error_setg(errp, "Cannot throttle device with multiple parents");
-        goto out;
-    }
-
     throttle_config_init(&cfg);
     cfg.buckets[THROTTLE_BPS_TOTAL].avg = bps;
     cfg.buckets[THROTTLE_BPS_READ].avg  = bps_rd;