tests: virtio-9p: ".." cannot be used to walk out of the shared directory

According to the 9P spec at http://man.cat-v.org/plan_9/5/intro, the
parent directory of the root directory of a server's tree is itself.
This test hence checks that the qid of the root directory as returned by
attach is the same as the qid of ".." when walking from the root directory.

Signed-off-by: Greg Kurz <groug@kaod.org>
