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>
diff --git a/tests/qemu-iotests/019.out b/tests/qemu-iotests/019.out
index 241d30c..b51224b 100644
--- a/tests/qemu-iotests/019.out
+++ b/tests/qemu-iotests/019.out
@@ -1,5 +1,5 @@
 QA output created by 019
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 cluster_size=65536 
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 
 Filling base image
 
 === IO: pattern 42
@@ -269,7 +269,7 @@
 qemu-io> No errors were found on the image.
 Creating test image with backing file
 
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file='TEST_DIR/t.IMGFMT.base' cluster_size=65536 
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file='TEST_DIR/t.IMGFMT.base' 
 Filling test image
 
 === IO: pattern 43