i386/tdx: Disable SMM for TDX VMs

i386/tdx: Disable SMM for TDX VMs

TDX doesn't support SMM and VMM cannot emulate SMM for TDX VMs because
VMM cannot manipulate TDX VM's memory.

Disable SMM for TDX VMs and error out if user requests to enable SMM.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-37-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>