Can't convert to vmdk with the streamOptimized subformat

Hi all,
I'm trying to convert a qcow2 image file to the vmdk (on Ubuntu Server 12.04):

# qemu-img convert -f qcow2 -O vmdk -o Stream spv-2912.qcow2 spv-2912-3.vmdk -o subformat=streamOptimized
VMDK: can't write to allocated cluster for streamOptimized
qemu-img: error while writing sector 65: Input/output error

Same result with any input format. Others subformats work but the StreamOptimized it is by far the most important one. The only workaround I found is to migrate to raw and then to use VBoxManage (VirtualBox).