i386/tdx: Introduce is_tdx_vm() helper and cache tdx_guest object

i386/tdx: Introduce is_tdx_vm() helper and cache tdx_guest object

It will need special handling for TDX VMs all around the QEMU.
Introduce is_tdx_vm() helper to query if it's a TDX VM.

Cache tdx_guest object thus no need to cast from ms->cgs every time.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-7-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>