LoongArch: KVM: Add IPI user mode read and write function

Implement the communication interface between the user mode programs
and the kernel in IPI interrupt controller simulation, which is used
to obtain or send the simulation data of the interrupt controller in
the user mode process, and is also used in VM migration or VM saving
and restoration.

Signed-off-by: Min Zhou <zhoumin@loongson.cn>
Signed-off-by: Tianrui Zhao <zhaotianrui@loongson.cn>
Signed-off-by: Xianglai Li <lixianglai@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>