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
    '''
    pkg_re = re.compile(r'^{0}-[\d.-]+-{1}$'.format(
        _package_prefix(), _kernel_type()))
    pkgs = __salt__['pkg.list_pkgs'](versions_as_list=True)
    if pkgs is None:
        pkgs = []

    result = list(filter(pkg_re.match, pkgs))
    if result is None:
        return []

    prefix_len = len(_package_prefix()) + 1

    if six.PY2:
        return sorted([pkg[prefix_len:] for pkg in result], cmp=_cmp_version)
    else:
        return sorted([pkg[prefix_len:] for pkg in result], key=functools.cmp_to_key(_cmp_version))