nixos/libvirtd: provide path to cloud-hypervisor for virtchd.service

Otherwise, starting this daemon fails with the message

```
virtchd[3484224]: Initialization of mandatory cloud-hypervisor state driver skipped
virtchd[3484224]: Driver state initialization failed
```

Fixes #167850

nixos/modules/virtualisation/libvirtd.nix

@@ -344,6 +344,10 @@ in
       restartIfChanged = false;
     };
 
+    systemd.services.virtchd = {
+      path = [ pkgs.cloud-hypervisor ];
+    };
+
     systemd.services.libvirt-guests = {
       wantedBy = [ "multi-user.target" ];
       path = with pkgs; [ coreutils gawk cfg.package ];