New for subnodes: the host of a subnode now automatically gets a
desire for 'hosts-<type>', where <type> is the type of its child node. This helps assign, because it can now limit the number of places to try assigning the host, and it means that we can give the hosts this feature, so that they don't get used for other purposes. For example, we can give the IXP-hosting nodes the feature 'hosts-ixp-bv' with weight 1, and they will never get used for anything but IXP hosting. This means that the node_type_features (or just node_features) table must now have hosts-<type> entires to work correctly.