tests: virtio-9p: add walk operation test

tests: virtio-9p: add walk operation test

The walk operation is used to traverse the directory tree and to associate
paths to fids. A single walk can be used to traverse up to P9_MAXWELEM path
elements at the same time.

The test creates a path with P9_MAXWELEM elements on the backend (à la
'mkdir -p') and issues a walk operation. The walk is expected to succeed
without error.

Reference:

http://man.cat-v.org/plan_9/5/walk

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