physical: Refactor port binding processing.

Extract block from SBREC_PORT_BINDING_FOR_EACH block in
physical_run to helper method so that it can be reused when
doing incremental processing.

Side effects:
  - localvif_to_oport and tunnels are now static file scoped.
  - the is_new parameter is added for use in a later patch set.

Signed-off-by: Ryan Moats <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>