OvmfPkg/QemuFwCfgLib: Support Tdx in QemuFwCfgDxe

OvmfPkg/QemuFwCfgLib: Support Tdx in QemuFwCfgDxe

RFC: https://bugzilla.tianocore.org/show_bug.cgi?id=3429

In the previous QemuFwCfgDxe only SEV is supported. This commit
introduce TDX support in QemuFwCfgDxe.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>