tests: Fix mocking of open()

tests: Fix mocking of open()

In some cases (well, majority), open() is either rewritten to
open64(), either by plain '#define open open64') or at assembly
level (using __REDIRECT macro). See <fcntl.h> for more info.

This didn't really matter to us, because we do not chain load two
mocks that would need to reimplement open() at the same time. But
this is soon going to change.

The problem is, that VIR_MOCK_REAL_INIT(open) glances over
aforementioned rewrite and initializes real_open pointer to
open() from the standard C library. But it needs to point to
open() (well, open64()) from the next mock on the list.

Therefore, init real_open to open64().

But of course, this is all glibc specific and for example musl
does the oposite (#define open64 open).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>