if you are using kvm/qemu with libvirt you can try to delete the
snapshot with the qemu monitor commands.
The general syntax is
virsh qemu-monitor-command --hmp <domain> '<command> [...]'
So you can try
virsh qemu-monitor-command --hmp DOMINO1-prealloc 'info snapshots'
and thne delete it with the provided snapshot ID
virsh qemu-monitor-command --hmp DOMINO1-prealloc 'delvm <ID>'
This will delete the snapshot from your diskimage.
Thanks Christian; this works too on a system with newer libvirt, but on my
old ubuntu 10.10 this command is unknown, so I had to resort to Python to
solve my problem. But it is solved :-).
Grts, J