LoongArch: KVM: Add kvm related header files

Add LoongArch KVM related header files, including kvm.h, kvm_host.h and
kvm_types.h. All of those are about LoongArch virtualization features
and kvm interfaces.

Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Tested-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Tianrui Zhao <zhaotianrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>