i386: Introduce tdx-guest object

i386: Introduce tdx-guest object

Introduce tdx-guest object which inherits X86_CONFIDENTIAL_GUEST,
and will be used to create TDX VMs (TDs) by

  qemu -machine ...,confidential-guest-support=tdx0	\
       -object tdx-guest,id=tdx0

It has one QAPI member 'attributes' defined, which allows user to set
TD's attributes directly.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-3-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>