hw/loongarch/virt: Implement cpu unplug interface

Implement cpu unplug interfaces including virt_cpu_unplug_request()
and virt_cpu_unplug().

Co-developed-by: Xianglai Li <lixianglai@loongson.cn>
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
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