ofproto-dpif-xlate: Add xlate_normal_flood()

This is also needed for multicast snooping.

Acked-by: Thomas Graf <tgraf@redhat.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>