vmm: Enforce threads_per_core to be at most two

Microsoft Hypervisor's hyperclear technology supports at most two
threads per core.

Practically all x64 CPUs nowadays only support two threads per core.
Enforce this in the common code.

Assisted-by: OpenAI:ChatGPT-5.4
[Test cases written by an LLM ]
Signed-off-by: Wei Liu <liuwe@microsoft.com>

cloud-hypervisor/tests/integration.rs

@@ -66,6 +66,7 @@ mod common_parallel {
     }
 
     #[test]
+    #[cfg_attr(target_arch = "x86_64", should_panic)]
     fn test_cpu_topology_421() {
         test_cpu_topology(4, 2, 1, false);
     }