Add media instance to media list.
        
        The L{lock} should be held upon entering this function.
        @param mrl: a media instance or a MRL.
        @return: 0 on success, -1 if the media list is read-only.

def add_media(self, mrl):
        """Add media instance to media list.
        
        The L{lock} should be held upon entering this function.
        @param mrl: a media instance or a MRL.
        @return: 0 on success, -1 if the media list is read-only.
        """
        if isinstance(mrl, basestring):
            mrl = (self.get_instance() or get_default_instance()).media_new(mrl)
        return libvlc_media_list_add_media(self, mrl)