LoongArch: KVM: Make kvm_get_vcpu_by_cpuid() more robust

In the Linux kernel, the following vulnerability has been resolved:

LoongArch: KVM: Make kvm_get_vcpu_by_cpuid() more robust

kvm_get_vcpu_by_cpuid() takes a cpuid parameter whose type is int, so

cpuid can be negative. Let kvm_get_vcpu_by_cpuid() return NULL for this

case so as to make it more robust.

This fix an out-of-bounds access to kvm_arch::phyid_map::phys_map[].