Vlv2TbltDevicePkg: Convert the device GUIDs of capsules to lowercase

LVFS assumes the GUIDs in the metainfo.xml are lowercase, so fwupdmgr
may fail to match the devices. Convert the GUIDs to lowercase to be
compatible with LVFS.

Cc: Zailiang Sun <zailiang.sun@intel.com>
Cc: Yi Qian <yi.qian@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>