Compress an raw image into qcow2

When creating an snapshot of a running instance that will serve as a new image for future intances.
Then it would be a really good feature to be able to shrink the snapshot size down to the actual diskspace used.
This can be done by hand for linux images, by doing as follows:
qemu-img convert -c input-image -O qcow2 outpu-image

So it would be really nice if this could be done through horizon