Make use of PERF_COUNT_HW_REF_CPU_CYCLES conditional
    
    The PERF_COUNT_HW_REF_CPU_CYCLES constant is not available
    on all Linux distros libvirt targets, so its use must be
    made conditional. Other constant have existed long enough
    that we can assume they exist, as we don't support very
    old distros like RHEL-5 any more.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>