passt: rename tap_sock_init() to tap_backend_init()

Extract pool storage initialization loop to tap_sock_update_pool(),
extract QEMU hints to tap_backend_show_hints().

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

passt.c

@@ -244,7 +244,7 @@ int main(int argc, char **argv)
 
 	pasta_netns_quit_init(&c);
 
-	tap_sock_init(&c);
+	tap_backend_init(&c);
 
 	random_init(&c);