Find out which of the given flags are currently not set.
    CLI Example:

    .. code-block:: bash

        salt '*' portage_config.get_missing_flags use salt "['ldap', '-libvirt', 'openssl']"

def get_missing_flags(conf, atom, flags):
    '''
    Find out which of the given flags are currently not set.
    CLI Example:

    .. code-block:: bash

        salt '*' portage_config.get_missing_flags use salt "['ldap', '-libvirt', 'openssl']"
    '''
    new_flags = []
    for flag in flags:
        if not has_flag(conf, atom, flag):
            new_flags.append(flag)
    return new_flags