hw/intc/arm_gicv3_common: Give no-migration-shift-bug subsection a needed function

Currently the migration code incorrectly treats a subsection with
no .needed function pointer as if it was the subsection list
terminator -- it is ignored and so is everything after it.
Work around this by giving vmstate_gicv3_gicd_no_migration_shift_bug
a 'needed' function that always returns true.

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: 20180806123445.1459-2-peter.maydell@linaro.org