On my TW I'm using latest virt-manager-2.0.0-1.1. In the past when I've created
a new VM the space for qcow2 image was allocated dynamically.

But nowadays virt-manager is creating a non-sparse qcow2 images by default so
it allocates lot of space on my rootfs.

As a workaround one can convert existing non-sparse image into sparse by
executing:
> qemu-img convert -O qcow2 original.qcow2 original_sparse.qcow2

And then switch the used disk image to the new one by "virsh edit" and editing
//devices/disk/source[@file="original_sparse.qcow2"]

How can we switch back to the default sparse qcow2 image creation method?