hw/loongarch/virt: Set max 256 cpus support on loongarch virt machine

hw/loongarch/virt: Set max 256 cpus support on loongarch virt machine

Add separate macro EXTIOI_CPUS for extioi interrupt controller, extioi
only supports 4 cpu. And set macro LOONGARCH_MAX_CPUS as 256 so that
loongarch virt machine supports more cpus.

Interrupts from external devices can only be routed cpu 0-3 because
of extioi limits, cpu internal interrupt such as timer/ipi can be
triggered on all cpus.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20230512100421.1867848-3-gaosong@loongson.cn>