OvmfPkg/AmdSev: assign and reserve the Sev Secret area

Create a one page secret area in the MEMFD and reserve the area with a
boot time HOB.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3077
Signed-off-by: James Bottomley <jejb@linux.ibm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201130202819.3910-6-jejb@linux.ibm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
[lersek@redhat.com: s/protect/reserve/g in the commit message, at Ard's
 and James's suggestion]

OvmfPkg/AmdSev/SecretPei/SecretPei.c

@@ -0,0 +1,25 @@
+/** @file
+  SEV Secret boot time HOB placement
+
+  Copyright (C) 2020 James Bottomley, IBM Corporation.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+#include <PiPei.h>
+#include <Library/HobLib.h>
+#include <Library/PcdLib.h>
+
+EFI_STATUS
+EFIAPI
+InitializeSecretPei (
+  IN       EFI_PEI_FILE_HANDLE  FileHandle,
+  IN CONST EFI_PEI_SERVICES     **PeiServices
+  )
+{
+  BuildMemoryAllocationHob (
+    PcdGet32 (PcdSevLaunchSecretBase),
+    PcdGet32 (PcdSevLaunchSecretSize),
+    EfiBootServicesData
+    );
+
+  return EFI_SUCCESS;
+}