When using self-service mode and trying to link with a remote matrix
room (room not in the same HS as the bridge user), then we need to add
the "via" HSs to join the room with, or else it fails.
We get it from the "m.space.child" in the "children_state" of the space
hierarchy.
It seems like the "via" information can also be stored in the
"m.space.parent" in the states of the room, but hopefully this shouldn't
be needed in sane implementations
