usbip: allow configuring script snippet to get ip address

modules/usbip.nix

@@ -17,6 +17,15 @@ in
       example = [ "4-1" ];
       description = "Auto attach devices with provided Bus IDs.";
     };
+    snippetIpAddress = {
+      type = lib.types.str;
+      default = "$(ip route list | sed -nE 's/(default)? via (.*) dev eth0 proto kernel/\2/p')";
+      example = "127.0.0.1";
+      description = ''
+        This snippet is used to obtain the address of the Windows host where Usbipd is running.
+        It can also be a plain IP address in case networkingMode=mirrored or wsl-vpnkit is used.
+      '';
+    };
   };
 
   config = lib.mkIf (config.wsl.enable && cfg.enable) {
@@ -39,7 +48,7 @@ in
 
         script = ''
           busid="$1"
-          ip="$(ip route list | sed -nE 's/(default)? via (.*) dev eth0 proto kernel/\2/p')"
+          ip="${cfg.snippetIpAddress}"
 
           echo "Starting auto attach for busid $busid on $ip."
           source ${usbipd-win-auto-attach} "$ip" "$busid"