tests: Lookup extended stat/lstat in mocks

tests: Lookup extended stat/lstat in mocks

macOS syscall interface (/usr/lib/system/libsystem_kernel.dylib) has
three kinds of stat but only one of them can be used to fill
"struct stat": stat$INODE64.

virmockstathelpers looks up regular stat instead of stat$INODE64.  That
causes a failure in qemufirmwaretest because "struct stat" is laid out
differently from the values returned by stat.

Introduce VIR_MOCK_REAL_INIT_ALIASED that can be used to lookup
stat$INODE64 and lstat$INODE64 and use it to setup real functions on
macOS.

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>