network: add TUN/TAP vt-* network rule for VMs

VM typically use a TAP device, and work similarly to a veth device from the
host side.

network/80-vm-vt.network

@@ -0,0 +1,24 @@
+#  SPDX-License-Identifier: LGPL-2.1+
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+# This network file matches vt-* TUN/TAP devices and applies a similar
+# configuration as ve-* to provide NAT/DHCP to VMs.
+
+[Match]
+Name=vt-*
+Driver=tun
+
+[Network]
+# Default to using a /28 prefix, giving up to 13 addresses per VM.
+Address=0.0.0.0/28
+LinkLocalAddressing=yes
+DHCPServer=yes
+IPMasquerade=yes
+LLDP=yes
+EmitLLDP=customer-bridge

network/meson.build

@@ -4,6 +4,7 @@ if conf.get('ENABLE_NETWORKD') == 1
         install_data('80-container-host0.network',
                      '80-container-ve.network',
                      '80-container-vz.network',
+                     '80-vm-vt.network',
                      '80-wifi-adhoc.network',
                      '80-wifi-ap.network.example',
                      '80-wifi-station.network.example',