networkd: introduce netdev "Netdevsim" Driver

This "netdevsim" as implied by the name is a tool for network developers and is a simulator.
This simulated networking device is used for testing various networking APIs and at this time
is particularly focused on testing hardware offloading related interfaces.

src/network/netdev/netdevsim.c

@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+/***
+  This file is part of systemd.
+
+  Copyright 2018 Susant Sahani
+***/
+
+#include "netdev/netdevsim.h"
+#include "missing.h"
+
+const NetDevVTable netdevsim_vtable = {
+        .object_size = sizeof(NetDevSim),
+        .sections = "Match\0NetDev\0",
+        .create_type = NETDEV_CREATE_INDEPENDENT,
+};