OvmfPkg/PlatformPei: Refactor InitializeRamRegions

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3863

InitializeRamRegions is refactored into 3 calls:
 - PlatformQemuInitializeRam
 - SevInitializeRam
 - PlatformQemuInitializeRamForS3

SevInitializeRam is not in PlatformInitLib. Because in the first stage
PlatformInitLib only support the basic platform featues.

PlatformQemuInitializeRamForS3 wraps the code which was previously in
InitializeRamRegions (many code in 2 if-checks).

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Sebastien Boeuf <sebastien.boeuf@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>

OvmfPkg/PlatformPei/Platform.h

@@ -35,7 +35,8 @@ PublishPeiMemory (
   );
 
 UINT32
-GetSystemMemorySizeBelow4gb (
+EFIAPI
+PlatformGetSystemMemorySizeBelow4gb (
   IN EFI_HOB_PLATFORM_INFO  *PlatformInfoHob
   );