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>

cloud-hypervisor/src/main.rs

@@ -679,7 +679,6 @@ fn start_vmm(
             monitor,
             &seccomp_action,
             landlock_enable,
-            hypervisor.hypervisor_type(),
             exit_evt.try_clone().unwrap(),
         )
         .map_err(Error::EventMonitorThread)?;