OvmfPkg: Check Tdx in QemuFwCfgPei to avoid DMA operation

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

If TDX is enabled then we do not support DMA operation in PEI phase.
This is mainly because DMA in TDX guest requires using bounce buffer
(which need to allocate dynamic memory and allocating a PAGE size'd
buffer can be challenge in PEI phase).

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>

OvmfPkg/Library/QemuFwCfgLib/QemuFwCfgLibInternal.h

@@ -59,4 +59,15 @@ InternalQemuFwCfgDmaBytes (
   IN     UINT32  Control
   );
 
+/**
+  Check if it is Tdx guest
+
+  @retval    TRUE   It is Tdx guest
+  @retval    FALSE  It is not Tdx guest
+**/
+BOOLEAN
+QemuFwCfgIsTdxGuest (
+  VOID
+  );
+
 #endif