in nova/virt/libvirt/utils.py

def create_cow_image(backing_file, path):
    """Create COW image

    Creates a COW image with the given backing file

    :param backing_file: Existing image on which to base the COW image
    :param path: Desired location of the COW image
    """
    execute('qemu-img', 'create', '-f', 'qcow2', '-o',
             'backing_file=%s' % backing_file, path)