iotests: add LUKS payload overhead to 178 qemu-img measure test

The previous patch includes the LUKS payload overhead into the qemu-img
measure calculation for qcow2.  Update qemu-iotests 178 to exercise this
new code path.

Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190218104525.23674-3-stefanha@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>