Supplement kvm document about LoongArch-specific part, such as add
api introduction for GET/SET_ONE_REG, GET/SET_FPU, GET/SET_MP_STATE,
etc.
Reviewed-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Tianrui Zhao <zhaotianrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>