CVE-2026-31558 LoongArch: KVM: Make kvm_get_vcpu_by_cpuid() more robust

In the Linux kernel, the following vulnerability has been resolved: LoongArch: KVM: Make kvmgetvcpubycpuid more robust kvmgetvcpubycpuid takes a cpuid parameter whose type is int, so cpuid can be negative. Let kvmgetvcpubycpuid return NULL for this case so as to make it more robust. This fix an...