virtualbox service: hide vboxnet0 from NetworkManager, fixes #10862

nixos/modules/virtualisation/virtualbox-host.nix

···
       };
 
     networking.interfaces.vboxnet0.ip4 = [ { address = "192.168.56.1"; prefixLength = 24; } ];
+    # Make sure NetworkManager won't assume this interface being up
+    # means we have internet access.
+    networking.networkmanager.unmanaged = ["vboxnet0"];
   })]);
 }