i386/tdx: Implement tdx_kvm_init() to initialize TDX VM context

i386/tdx: Implement tdx_kvm_init() to initialize TDX VM context

Implement TDX specific ConfidentialGuestSupportClass::kvm_init()
callback, tdx_kvm_init().

Mark guest state is proctected for TDX VM.  More TDX specific
initialization will be added later.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-5-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>