iotests: Prefer null-co over null-aio

We use null-co basically everywhere in the iotests.  Unless we want to
test null-aio specifically, we should use it instead (for consistency).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20190917092004.999-2-mreitz@redhat.com
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
2 files changed