fix: add required max_vcpus to cloud-hypervisor cpus config

cloud-hypervisor's CpusConfig requires max_vcpus when the cpus object
is present in JSON. Set it equal to boot_vcpus (no CPU hotplug).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/scripts.nix

@@ -946,6 +946,7 @@ let
         };
         cpus = {
           boot_vcpus = vm.cpus;
+          max_vcpus = vm.cpus;
           core_scheduling = coreScheduling;
         };
         balloon = {