virmock.h: Introduce VIR_MOCK_CALL_STAT
    
    There is some magic going on when it comes to stat() or lstat().
    Basically, stat() can either be a regular function, an inline
    function that calls __xstat(_STAT_VER, ...) or a macro that does
    the same as the inline func. Don't ask why is that, just read the
    documentation in sys/stat.h and make sure you have a bucket next
    to you. Anyway, currently there will not be both stat and __xstat
    symbols at the same time, as one of them gets overwritten to the
    other one during compilation. But this is not true anymore once
    we start chaining our mocking libraries. Therefore we need a
    wrapper that calls desired function from glibc.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>