hw/intc/arm_gicv3_kvm: Add ICC_SRE_EL1 register to vmstate

To Save and Restore ICC_SRE_EL1 register introduce vmstate
subsection and load only if non-zero.
Also initialize icc_sre_el1 with to 0x7 in pre_load
function.

Signed-off-by: Vijaya Kumar K <Vijaya.Kumar@cavium.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-id: 1487850673-26455-3-git-send-email-vijay.kilari@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>