LoongArch: KVM: Implement kvm exception vectors

Implement kvm exception vectors, using kvm_fault_tables array to save
the handle function pointers and it is used when vcpu handle guest exit.

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>