dpif: New function dpif_get_netflow_ids().

dpif: New function dpif_get_netflow_ids().

The 'minor' member of struct dpif is used for two different purposes:
for printing in log messages and for encapsulating in NetFlow messages.
The needs in each case are different, so we should break up these uses.
This commit does half of that, by introducing a new function to retrieve
NetFlow ids and using it where appropriate.