OvmfPkg/PlatformInitLib: Add PlatformGetLowMemoryCB

Add PlatformGetLowMemoryCB() callback function for use with
PlatformScanE820().  It stores the low memory size in
PlatformInfoHob->LowMemory.  This replaces calls to
PlatformScanOrAdd64BitE820Ram() with non-NULL LowMemory.

Write any actions done (setting LowMemory) to the firmware log
with INFO loglevel.

Also change PlatformGetSystemMemorySizeBelow4gb() to likewise set
PlatformInfoHob->LowMemory instead of returning the value.  Update
all Callers to the new convention.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/Include/Library/PlatformInitLib.h

@@ -26,6 +26,7 @@ typedef struct {
   BOOLEAN              Q35SmramAtDefaultSmbase;
   UINT16               Q35TsegMbytes;
 
+  UINT32               LowMemory;
   UINT64               FirstNonAddress;
   UINT8                PhysMemAddressWidth;
   UINT32               Uc32Base;
@@ -144,7 +145,7 @@ PlatformQemuUc32BaseInitialization (
   IN OUT EFI_HOB_PLATFORM_INFO  *PlatformInfoHob
   );
 
-UINT32
+VOID
 EFIAPI
 PlatformGetSystemMemorySizeBelow4gb (
   IN EFI_HOB_PLATFORM_INFO  *PlatformInfoHob