Unfortunately this patch is not safe: theoretically ->attach can call
bdrv_apply_subtree_drain, and if it polls, will can call a bh that
for example reads the graph, finding it in an inconsistent state, since
it is between the two writes QLIST_INSERT_HEAD(&bs->children, child,
next); and QLIST_INSE
Doing the opposite can make adding the child node to a non-drained node,
as apply_subtree_drain is only done in ->attach() and thus make
assert_bdrv_graph_writable fail.
This can happen for example during a transaction rollback (test 245,
test_io_with_graph_changes):
1. a node is removed from the
