"internal" netdevs are treated specially in OVS (e.g. for MTU), but
the dummy datapath remaps both "system" and "internal" devices to the
same "dummy" netdev class, so there's no way to discern those in tests.

This commit adds a new "dummy-internal" netdev type, which will be used
by the dummy datapath for internal ports, so that other parts of the
code can understand which ports are internal just by looking at the
netdev object.

The alternative solution, using the original interface type ("internal")
instead of the translated netdev type ("dummy"), is harder to implement,
because in so many places only the netdev object is available.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ben Pfaff <blp@ovn.org>