KVM: selftests: Use X86_PROPERTY_MAX_KVM_LEAF in CPUID test

Use X86_PROPERTY_MAX_KVM_LEAF to replace the equivalent open coded check
on KVM's maximum paravirt CPUID leaf.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221006005125.680782-5-seanjc@google.com