process: Translate "unlimited" correctly
    
    The value we use internally to represent the lack of a memory
    locking limit, VIR_DOMAIN_MEMORY_PARAM_UNLIMITED, doesn't
    match the value setrlimit() and prlimit() use for the same
    purpose, RLIM_INFINITY, so we have to handle the translation
    ourselves.
    
    Partially-resolves: https://bugzilla.redhat.com/1431793