Add functions to determine how port should be opened based on type.

Depending on the port and type of datapath, a port may need to be opened
as a different type of device than it's configured.  For example, an
"internal" port on a "dummy" datapath should opened as a "dummy" port.
This commit adds the ability for a dpif to provide this information to a
caller.  It will be used in a future commit.

Signed-off-by: Justin Pettit <jpettit@nicira.com>

lib/dpif-linux.c

@@ -1281,6 +1281,7 @@ dpif_linux_recv_purge(struct dpif *dpif_)
 const struct dpif_class dpif_linux_class = {
     "system",
     dpif_linux_enumerate,
+    NULL,
     dpif_linux_open,
     dpif_linux_close,
     dpif_linux_destroy,