Keep requesting all attributes until it works.

        Immediately after a power on event (POW1) the AVR is inconsistent with
        which attributes can be successfully queried.  When we detect that
        power has just been turned on, we loop every second making a bulk
        query for every known attribute.  This continues until we detect that
        values have been returned for at least one input name (this seems to
        be the laggiest of all the attributes)

def poweron_refresh(self):
        """Keep requesting all attributes until it works.

        Immediately after a power on event (POW1) the AVR is inconsistent with
        which attributes can be successfully queried.  When we detect that
        power has just been turned on, we loop every second making a bulk
        query for every known attribute.  This continues until we detect that
        values have been returned for at least one input name (this seems to
        be the laggiest of all the attributes)
        """
        if self._poweron_refresh_successful:
            return
        else:
            self.refresh_all()
            self._loop.call_later(2, self.poweron_refresh)