perf: get rid of pointless virPerfGetEvent() method
    
    The virPerfGetEvent method pointlessly checks for a NULL
    parameter and the range of an enum value. The whole point
    of using an enum is that we can avoid such checks. Just
    replace calls to virPerfGetEvent, with perf->events[type]
    array access.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>