KVM: selftests: Rename kvm_get_supported_cpuid_index() to __..._entry()

Rename kvm_get_supported_cpuid_index() to __kvm_get_supported_cpuid_entry()
to better show its relationship to kvm_get_supported_cpuid_entry(), and
because the helper returns a CPUID entry, not the index of an entry.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-37-seanjc@google.com