fix: Only create darwin examples for hypervisors that work on macOS

Most hypervisors (firecracker, cloud-hypervisor, crosvm, kvmtool,
stratovirt, alioth) require Linux KVM and cannot run on macOS.

Only qemu (via Apple's HVF) and vfkit work on darwin, so only create
example configurations for these two hypervisors on darwin systems.

This removes 24 misleading example configurations that could be built
but never run on macOS:
- aarch64-darwin-{alioth,cloud-hypervisor,crosvm,firecracker,kvmtool,stratovirt}-*
- x86_64-darwin-{alioth,cloud-hypervisor,crosvm,firecracker,kvmtool,stratovirt}-*

Net result:
- Added: 2 vfkit examples (aarch64-darwin, x86_64-darwin)
- Removed: 24 KVM-only darwin examples
- Kept: All Linux examples + qemu darwin examples (HVF)

Author: luizribeiro

flake.nix

@@ -167,6 +167,8 @@
             ];
             hypervisorsWithUserNet = [ "qemu" "kvmtool" "vfkit" ];
             hypervisorsDarwinOnly = [ "vfkit" ];
+            # Hypervisors that work on darwin (qemu via HVF, vfkit natively)
+            hypervisorsOnDarwin = [ "qemu" "vfkit" ];
             hypervisorsWithTap = builtins.filter
               # vfkit supports networking, but does not support tap
               (hv: hv != "vfkit")
@@ -175,8 +177,9 @@
             isDarwinOnly = hypervisor: builtins.elem hypervisor hypervisorsDarwinOnly;
             isDarwinSystem = system: lib.hasSuffix "-darwin" system;
             hypervisorSupportsSystem = hypervisor: system:
-              # Darwin-only hypervisors only work on darwin, others work everywhere
-              !(isDarwinOnly hypervisor && !(isDarwinSystem system));
+              if isDarwinSystem system
+              then builtins.elem hypervisor hypervisorsOnDarwin
+              else !(isDarwinOnly hypervisor);
 
             makeExample = { system, hypervisor, config ? {} }:
               lib.nixosSystem {