OvmfPkg/BaseMemEncryptSevLib: Use AmdSvsmSnpPvalidate() to validate pages

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

The PVALIDATE instruction is used to change the SNP validation of a page,
but that can only be done when running at VMPL0. To prepare for running at
a less priviledged VMPL, use the AmdSvsmLib library API to perform the
PVALIDATE. The AmdSvsmLib library will perform the proper operation on
behalf of the caller.

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>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>

OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf

@@ -1,7 +1,7 @@
 ## @file
 #  Library provides the helper functions for SEV guest
 #
-# Copyright (c) 2020 Advanced Micro Devices. All rights reserved.<BR>
+# Copyright (c) 2020 - 2024, Advanced Micro Devices. All rights reserved.<BR>
 #
 #  SPDX-License-Identifier: BSD-2-Clause-Patent
 #
@@ -52,6 +52,7 @@
   MemoryAllocationLib
   PcdLib
   CcExitLib
+  AmdSvsmLib
 
 [FeaturePcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire