Comment 17 for bug 691609

As you'd observed from the start, I suspect we need to drop the 'format=raw'
from the command. Please add the following to the disk definition in your
.xml file (i.e. virsh edit a1.6-q2.log):

      <driver name='qemu' type='qcow2'/>

So the whole disk definition will probably look like:

<disk type="file" device="disk">
 <source file="/var/kvm/a1.6-q2.img"/>
 <driver name='qemu' type='qcow2'/>
 <target dev="hda" bus="ide"/>
</disk>