network: call proper start/stop functions for macvtap bridge-mode networks

network: call proper start/stop functions for macvtap bridge-mode networks

networkStartNetwork() and networkShutdownNetwork() were calling the
wrong type-specific function in the case of networks that were
configured for macvtap ("direct") bridge mode - they were instead
calling the functions for a tap+bridge network. Currently none of
these functions does anything (they just return 0) so it hasn't
created any problems, but that could change in the future.