Test that qemu-img convert -c works when input image length is not a
multiple of the cluster size.

Previously an error message would be produced:

  qemu-img: error while compressing sector 0: Input/output error

Now that qcow2 and qcow handle this case the test passes successfully.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>