qemu-iotests: ignore leaks on failure paths in 026

qemu-iotests: ignore leaks on failure paths in 026

Upcoming asynchronous handling of sub-parts of qcow2 requests will
change number of leaked clusters and even make it racy. As a
preparation, ignore leaks on failure parts in 026.

It's not trivial to just grep or substitute qemu-img output for such
thing. Instead do better: 3 is a error code of qemu-img check, if only
leaks are found. Catch this case and print success output.

Suggested-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190916175324.18478-2-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>