vmm: simplify seccomp code

Only Thread::Vmm and Thread::Vcpu need to know the hypervisor type.
Make the type optional, and then simplify the users.

Assisted-by: Pi-agent:Claude-Opus-4.7
Signed-off-by: Wei Liu <liuwe@microsoft.com>

vmm/src/cpu.rs

@@ -1172,7 +1172,7 @@ impl CpuManager {
         let vcpu_seccomp_filter = get_seccomp_filter(
             &self.seccomp_action,
             Thread::Vcpu,
-            self.hypervisor.hypervisor_type(),
+            Some(self.hypervisor.hypervisor_type()),
         )
         .map_err(Error::CreateSeccompFilter)?;