Enable extended L2 entries, reduce cluster size

* I/O perf should improve thanks to extended L2:
https://blogs.igalia.com/berto/2020/12/03/subcluster-allocation-for-qcow2-images/

* Decreasing cluster size produces smaller images

build-steps.d/1800_copy_vms_into_raw

@@ -43,7 +43,8 @@ copy-vm-files(){
 #       convert \
 #          -p \
 #          -O qcow2 \
-#          -o cluster_size=2M \
+#          -o extended_l2=on \
+#          -o cluster_size=128k \
 #          -o preallocation=metadata \
 #          "$binary_image_raw" \
 #          "$binary_image_qcow2"