Slight test speedup

Don't do a pointless ARP check in dhcpcd.

nixos/modules/services/networking/dhcpcd.nix

@@ -80,6 +80,7 @@ in
   options = {
 
     networking.dhcpcd.denyInterfaces = mkOption {
+      type = types.listOf types.str;
       default = [];
       description = ''
          Disable the DHCP client for any interface whose name matches
@@ -90,6 +91,7 @@ in
     };
 
     networking.dhcpcd.extraConfig = mkOption {
+      type = types.lines;
       default = "";
       description = ''
          Literal string to append to the config file generated for dhcpcd.

nixos/modules/virtualisation/qemu-vm.nix

@@ -399,6 +399,9 @@ in
     # Wireless won't work in the VM.
     networking.wireless.enable = mkVMOverride false;
 
+    # Speed up booting by not waiting for ARP.
+    networking.dhcpcd.extraConfig = "noarp";
+
     system.requiredKernelConfig = with config.lib.kernelConfig;
       [ (isEnabled "VIRTIO_BLK")
         (isEnabled "VIRTIO_PCI")