qemu-iotests: Update filter for default cluster size

qemu-iotests: Update filter for default cluster size

Until recently, qemu-img create displayed cluster_size=0 for the default
cluster size. It is changed to display the real cluster size now, which results
in the cluster size not being filtered out any more.

If the cluster size is specified explicitly in CLUSTER_SIZE, keep the output,
and if using the default, filter it out. This mostly restores the old behaviour
of the test cases; test 015 must be fixed to use CLUSTER_SIZE instead of using
extra_img_options for it.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>