KVM: selftests: Convert vmx_pmu_caps_test to use X86_PROPERTY_*

Add X86_PROPERTY_PMU_VERSION and use it in vmx_pmu_caps_test to replace
open coded versions of the same functionality.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221006005125.680782-9-seanjc@google.com