CVE-2026-31569: LoongArch: KVM: Handle the case that EIOINTC's coremap is empty

In the Linux kernel, the following vulnerability has been resolved: LoongArch: KVM: Handle the case that EIOINTC's coremap is empty EIOINTC's coremap in eiointc_update_sw_coremap() can be empty, currently we get a cpuid with -1 in this case, but we actually need 0 because it's similar as the case that cpuid >= 4. This fix an out-of-bounds access to kvm_arch::phyid_map::phys_map[].