LoongArch: KVM: Implement arch-specific functions for guest perf

Three architecture specific functions are added for the guest perf
feature, they are kvm_arch_vcpu_in_kernel(), kvm_arch_vcpu_get_ip()
and kvm_arch_pmi_in_guest().

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>