qemu-iotests: Always filter cluster_size out in _make_test_img

qemu-iotests: Always filter cluster_size out in _make_test_img

Some image formats do have a cluster size, others don't, but there are
tests that work with both sets of images and currently we get failures
because the qemu-img create output doesn't mention the cluster size for
some formats.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>