qtest: Add qtest_add_data_func() wrapper function

It calls g_test_add_data_func() with a path supplemented by the
architecture, like qtest_add_func() does.

Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>