hw/intc/arm_gicv3_common: Move post_load hooks to top-level VMSD

Contrary to the the impression given in docs/devel/migration.rst,
the migration code does not run the pre_load hook for a
subsection unless the subsection appears on the wire, and so
this is not a place where you can set the default value for
state for the "subsection not present" case. Instead this needs
to be done in a pre_load hook for whatever is the parent VMSD
of the subsection.

We got this wrong in two of the subsection definitions in
the GICv3 migration structs; fix this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 20180806123445.1459-5-peter.maydell@linaro.org