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/Library/PlatformInitLib/PlatformInitLib.inf

@@ -58,6 +58,7 @@
 
 [Guids]
   gEfiSmmSmramMemoryGuid
+  gEfiAcpiVariableGuid
 
 [Pcd]
   gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress