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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def start(uri=None,
          tag_prefix='salt/engines/libvirt_events',
          filters=None):
    '''
    Listen to libvirt events and forward them to salt.

    :param uri: libvirt URI to listen on.
                Defaults to None to pick the first available local hypervisor
    :param tag_prefix: the begining of the salt event tag to use.
                       Defaults to 'salt/engines/libvirt_events'
    :param filters: the list of event of listen on. Defaults to 'all'
    '''
    if filters is None:
        filters = ['all']
    try:
        libvirt.virEventRegisterDefaultImpl()

        cnx = libvirt.openReadOnly(uri)
        log.debug('Opened libvirt uri: %s', cnx.getURI())

        callback_ids = {}
        all_filters = "all" in filters

        for obj, event_defs in CALLBACK_DEFS.items():
            for event, real_id in event_defs:
                event_filter = "/".join((obj, event))
                if event_filter not in filters and obj not in filters and not all_filters:
                    continue
                registered_id = _register_callback(cnx, tag_prefix,
                                                   obj, event, real_id)
                if registered_id:
                    _append_callback_id(callback_ids, obj, registered_id)

        exit_loop = False
        while not exit_loop:
            exit_loop = libvirt.virEventRunDefaultImpl() < 0
            log.debug('=== in the loop exit_loop %s ===', exit_loop)

    except Exception as err:  # pylint: disable=broad-except
        log.exception(err)
    finally:
        _callbacks_cleanup(cnx, callback_ids)
        _cleanup(cnx)