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>