Return `on_crash` setting from the named vm

    CLI Example:

    .. code-block:: bash

        salt '*' virt.get_on_crash <domain>

def _get_on_crash(dom):
    '''
    Return `on_crash` setting from the named vm

    CLI Example:

    .. code-block:: bash

        salt '*' virt.get_on_crash <domain>
    '''
    node = ElementTree.fromstring(get_xml(dom)).find('on_crash')
    return node.text if node is not None else ''