nixos/qemu-vm: make networking options configurable

nixos/modules/virtualisation/qemu-vm.nix

@@ -57,8 +57,7 @@ let
           -name ${vmName} \
           -m ${toString config.virtualisation.memorySize} \
           ${optionalString (pkgs.stdenv.system == "x86_64-linux") "-cpu kvm64"} \
-          -net nic,vlan=0,model=virtio \
+          ${concatStringsSep " " config.virtualisation.qemuNetworkingOptions} \
-          -net user,vlan=0''${QEMU_NET_OPTS:+,$QEMU_NET_OPTS} \
           -virtfs local,path=/nix/store,security_model=none,mount_tag=store \
           -virtfs local,path=$TMPDIR/xchg,security_model=none,mount_tag=xchg \
           -virtfs local,path=''${SHARED_DIR:-$TMPDIR/xchg},security_model=none,mount_tag=shared \
@@ -180,6 +179,23 @@ in
           '';
       };
 
+    virtualisation.qemuNetworkingOptions =
+      mkOption {
+        default = [
+          "-net nic,vlan=0,model=virtio"
+          "-net user,vlan=0\${QEMU_NET_OPTS:+,$QEMU_NET_OPTS}"
+        ];
+        type = types.listOf types.str;
+        description = ''
+          Networking-related command-line options that should be passed to qemu.
+          The default is to use userspace networking (slirp).
+
+          If you override this option, be adviced to keep
+          ''${QEMU_NET_OPTS:+,$QEMU_NET_OPTS} (as seen in the default)
+          to keep the default runtime behaviour.
+        '';
+      };
+
     virtualisation.graphics =
       mkOption {
         default = true;