vswitch: Avoid knowledge of details specific to Linux datapaths.

At startup, the vswitch needs to delete datapaths that are not configured
by the administrator.  Until now this was done by knowing the possible
names of Linux datapaths.  This commit cleans up by allowing each
datapath class to enumerate its existing datapaths and their names.

lib/dpif-netdev.c

@@ -1294,8 +1294,10 @@ const struct dpif_class dpif_netdev_class = {
     "netdev",
     dp_netdev_run,
     dp_netdev_wait,
+    NULL,                       /* enumerate */
     dpif_netdev_open,
     dpif_netdev_close,
+    NULL,                       /* get_all_names */
     dpif_netdev_delete,
     dpif_netdev_get_stats,
     dpif_netdev_get_drop_frags,