nixos/vm: enable qemu network bridge

nixos/vm.nix

@@ -1,8 +1,14 @@
+{ lib, ... }:
 {
   virtualisation.vmVariant.virtualisation = {
     cores = 2;
     diskSize = 8192;
     graphics = false;
     memorySize = 4096;
+
+    qemu.networkingOptions = lib.mkForce [
+      "-nic bridge,br=virbr0,id=hn0,model=virt-net-pci,helper=\${QEMU_BRIDGE_HELPER_PATH}"
+      "-device virtio-net-pci,netdev=hn0,id=nic1,\${QEMU_NET_OPTS:+,$QEMU_NET_OPTS}"
+    ];
   };
 }

shells/default.nix

@@ -5,6 +5,24 @@
     {
       devShells.default = pkgs.mkShell {
         name = "configuration.nix";
+
+        shellHook = ''
+          for path in \
+            /usr/lib/qemu/qemu-bridge-helper \
+            /run/wrappers/bin/qemu-bridge-helper
+          do
+            if [ -x "$path" ]; then
+              export QEMU_BRIDGE_HELPER_PATH="$path"
+              break
+            fi
+          done
+
+          if [ -z "$QEMU_BRIDGE_HELPER_PATH" ]; then
+            printf "\033[1;33m%s\033[0m\n" \
+              "WARN: 'qemu-bridge-helper' not found, make sure it is installed and the nix shell hook is looking for it" >&2
+          fi
+        '';
+
         packages = with pkgs; [
           bat
           cachix