virmock.h: Introduce VIR_MOCK_CALL_STAT

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>