hw/intc/loongarch_extioi: Add vmstate post_load support

hw/intc/loongarch_extioi: Add vmstate post_load support

There are elements sw_ipmap and sw_coremap, which is usd to speed
up irq injection flow. They are saved and restored in vmstate during
migration, indeed they can calculated from hw registers. Here
post_load is added for get sw_ipmap and sw_coremap from extioi hw
state.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20231215100333.3933632-5-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>