Dynamips.create_nio requires the node as first positional argument
(unlike every other manager which takes only nio_settings). The batch
handler now detects this via parameter-count inspection (3 vs 2) and
passes node when needed.
Also add Dynamips to _add_nio_binding dispatch: routers use
slot_add_nio_binding(slot, port, nio), switches/hubs fall back to
add_nio(nio, port_number).
Add tests covering Dynamips router dispatch, switch dispatch, and the
create_nio signature detection to prevent regression.