Return a list of all installed kernels.

    CLI Example:

    .. code-block:: bash

        salt '*' kernelpkg.list_installed

def list_installed():
    '''
    Return a list of all installed kernels.

    CLI Example:

    .. code-block:: bash

        salt '*' kernelpkg.list_installed
    '''
    result = __salt__['pkg.version'](_package_name(), versions_as_list=True)
    if result is None:
        return []

    if six.PY2:
        return sorted(result, cmp=_cmp_version)
    else:
        return sorted(result, key=functools.cmp_to_key(_cmp_version))