util: make it more robust to calculate timeout value
    
    When we change system clock to years ago, a certain CPU may use up 100% cputime.
    The reason is that in function virEventPollCalculateTimeout(), we assign the
    unsigned long long result to an INT variable,
            *timeout = then - now; // timeout is INT, and then/now are long long
            if (*timeout < 0)
                *timeout = 0;
    there's a chance that variable @then minus variable @now may be a very large number
    that overflows INT value expression, then *timeout will be negative and be assigned to 0.
    Next the 'poll' in function virEventPollRunOnce() will get into an 'endless' while loop there.
    thus, the cpu that virEventPollRunOnce() thread runs on will go up to 100%.
    
    Although as we discussed before in https://www.redhat.com/archives/libvir-list/2015-May/msg00400.html
    it should be prohibited to set-time while other applications are running, but it does
    seems to have no harm to make the codes more robust.
    
    Signed-off-by: Wang Yufei <james.wangyufei@huawei.com>
    Signed-off-by: Zhang Bo <oscar.zhangbo@huawei.com>