Switch from raw images to qcow2 format

This allows to use minimal copy-on-write clones of the base image as
guest images, which in turn saves a lot of disk space.

testing/stop-testing

@@ -19,7 +19,7 @@ for host in $STRONGSWANHOSTS
 do
 	log_action "Guest $host"
 	execute "virsh destroy $host"
-	rm -f $VIRTIMGSTORE/$host.img
+	rm -f $VIRTIMGSTORE/$host.qcow2
 done
 
 log_action "Removing kernel $KERNEL"