tests/virtio-9p: added readdir test

The first readdir test simply checks the amount of directory
entries returned by 9pfs server, according to the created amount
of virtual files on 9pfs synth driver side. Then the subsequent
readdir test also checks whether all directory entries have the
expected file names (as created on 9pfs synth driver side),
ignoring their precise order in result list though.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <e0b4402722a877178f8fb6a8ad7b64bb20150613.1579567020.git.qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Greg Kurz <groug@kaod.org>