nixos/hyperv: bail gracefully if device is missing

nixos/modules/virtualisation/hyperv-guest.nix

@@ -56,6 +56,8 @@ in {
     systemd = {
       packages = [ config.boot.kernelPackages.hyperv-daemons.lib ];
 
+      services.hv-vss.unitConfig.ConditionPathExists = [ "/dev/vmbus/hv_vss" ];
+
       targets.hyperv-daemons = {
         wantedBy = [ "multi-user.target" ];
       };