hw/intc/arm_gicv3_common: Combine duplicate .subsections in vmstate_gicv3_cpu

Commit 6692aac4 accidentally introduced a second initialization
of the .subsections field of vmstate_gicv3_cpu, instead of adding
the new subsection to the existing list. The effect of this was
probably that migration of GICv3 with virtualization enabled was
broken (or alternatively that migration of ICC_SRE_EL1 was broken,
depending on which of the two initializers the compiler used).
Combine the two into a single list.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180806123445.1459-3-peter.maydell@linaro.org