virutil: introduce helper functions for memory limits
    
    The first one is to truncate the memory limit to
    VIR_DOMAIN_MEMORY_PARAM_UNLIMITED if the value is greater and the second
    one is to decide whether the memory limit is set or not, unlimited means
    that it's not set.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>