xen: arm: arm64: Fix memory cloberring issues during VFP save restore.

xen: arm: arm64: Fix memory cloberring issues during VFP save restore.

This patch addresses memory cloberring issue mentioed by Julien Grall
with my earlier patch -
Commit Id: 712eb2e04da2cbcd9908f74ebd47c6df60d6d12f

Discussion related to this fix -
http://www.gossamer-threads.com/lists/xen/devel/316247

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
Acked-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>