OvmfPkg: Create a calling area used to communicate with the SVSM

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4654

An SVSM requires a calling area page whose address (CAA) is used by the
SVSM to communicate and process the SVSM request.

Add a pre-defined page area to the OvmfPkg and AmdSev packages and define
corresponding PCDs used to communicate the location and size of the area.
Keep the AmdSev package in sync with the OvmfPkg and adjust the AmdSev
launch and hash area memory locations.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Michael Roth <michael.roth@amd.com>
Cc: Min Xu <min.m.xu@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>

OvmfPkg/PlatformPei/AmdSev.c

@@ -555,5 +555,16 @@ SevInitializeRam (
       (UINT64)(UINTN)PcdGet32 (PcdOvmfCpuidSize),
       EfiReservedMemoryType
       );
+
+    //
+    // The calling area memory needs to be protected until the OS can create
+    // its own calling area. Mark it as EfiReservedMemoryType so that the
+    // guest firmware and OS do not use it as a system memory.
+    //
+    BuildMemoryAllocationHob (
+      (EFI_PHYSICAL_ADDRESS)(UINTN)PcdGet32 (PcdOvmfSecSvsmCaaBase),
+      (UINT64)(UINTN)PcdGet32 (PcdOvmfSecSvsmCaaSize),
+      EfiReservedMemoryType
+      );
   }
 }