fix bug: DeviceState.get_all_children doesn't return all the children

[zr4in]

>>> a = set([1, 2, 3])
>>> b = set([2, 3, 4])
>>> a.union(b)
{1, 2, 3, 4}
>>> a
{1, 2, 3}
>>> a = a.union(b)
>>> a
{1, 2, 3, 4}

A set won't be update after set.union(set) called.