virschematest: Replace g_lstat() with virFileIsLink()

virschematest: Replace g_lstat() with virFileIsLink()

Inside of virschematest.c there's testSchemaDir() which iterates
over dentries in given directory but skips some files: those
without ".xml" suffix, hidden files, symlinks, etc.

Now, symlinks are detected as g_lstat() + S_ISLNK() combo which
works, except it fails to compile on mingw where is no concept of
symlinks. Replace the combo with a call to virFileIsLink() which
at least allows us to compile cleanly on mingw.

Fixes: f997fcca71a16b102e6ee663a3fb86bed8de9d7d
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>