iotests/283: Use consistent size for source and target

iotests/283: Use consistent size for source and target

The test case forgot to specify the null-co size for the target node.
When adding a check to backup that both sizes match, this would fail
because of the size mismatch and not the behaviour that the test really
wanted to test.

Fixes: a541fcc27c98b96da187c7d4573f3270f3ddd283
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200430142755.315494-2-kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>