memtune: change the way how we store unlimited value
    
    There was a mess in the way how we store unlimited value for memory
    limits and how we handled values provided by user.  Internally there
    were two possible ways how to store unlimited value: as 0 value or as
    VIR_DOMAIN_MEMORY_PARAM_UNLIMITED.  Because we chose to store memory
    limits as unsigned long long, we cannot use -1 to represent unlimited.
    It's much easier for us to say that everything greater than
    VIR_DOMAIN_MEMORY_PARAM_UNLIMITED means unlimited and leave 0 as valid
    value despite that it makes no sense to set limit to 0.
    
    Remove unnecessary function virCompareLimitUlong.  The update of test
    is to prevent the 0 to be miss-used as unlimited in future.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1146539
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>