OvmfPkg: Refine SmmAccess implementation

This patch refines the SmmAccess implementation:
1. SmramMap will be retrieved from the
gEfiSmmSmramMemoryGuid instead of original from
the TSEG Memory Base register.
2. Remove the gEfiAcpiVariableGuid creation, thus
the DESCRIPTOR_INDEX definition can be also cleaned.
3. The gEfiAcpiVariableGuid HOB is moved to the
OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>

OvmfPkg/SmmAccess/SmmAccess2Dxe.inf

@@ -5,6 +5,7 @@
 # driver.
 #
 # Copyright (C) 2013, 2015, Red Hat, Inc.
+# Copyright (c) 2024 Intel Corporation.
 #
 # SPDX-License-Identifier: BSD-2-Clause-Patent
 #
@@ -41,6 +42,7 @@
   PciLib
   UefiBootServicesTableLib
   UefiDriverEntryPoint
+  HobLib
 
 [Protocols]
   gEfiSmmAccess2ProtocolGuid   ## PRODUCES
@@ -48,6 +50,9 @@
 [FeaturePcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
 
+[Guids]
+  gEfiSmmSmramMemoryGuid             # ALWAYS_CONSUMED
+
 [Pcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdQ35SmramAtDefaultSmbase
   gUefiOvmfPkgTokenSpaceGuid.PcdQ35TsegMbytes