Return available OpenNebula images.

    CLI Example:

    .. code-block:: bash

        salt-cloud --list-images opennebula
        salt-cloud --function avail_images opennebula
        salt-cloud -f avail_images opennebula

def avail_images(call=None):
    '''
    Return available OpenNebula images.

    CLI Example:

    .. code-block:: bash

        salt-cloud --list-images opennebula
        salt-cloud --function avail_images opennebula
        salt-cloud -f avail_images opennebula

    '''
    if call == 'action':
        raise SaltCloudSystemExit(
            'The avail_images function must be called with '
            '-f or --function, or with the --list-images option'
        )

    server, user, password = _get_xml_rpc()
    auth = ':'.join([user, password])

    image_pool = server.one.imagepool.info(auth, -2, -1, -1)[1]

    images = {}
    for image in _get_xml(image_pool):
        images[image.find('NAME').text] = _xml_to_dict(image)

    return images