get expected global unicast IPv6 address of OpenThreadWpan

        Args:
            filterByPrefix: a given expected global IPv6 prefix to be matched

        Returns:
            a global IPv6 address

def getGUA(self, filterByPrefix=None):
        """get expected global unicast IPv6 address of OpenThreadWpan

        Args:
            filterByPrefix: a given expected global IPv6 prefix to be matched

        Returns:
            a global IPv6 address
        """
        print '%s call getGUA' % self.port
        print filterByPrefix
        globalAddrs = []
        try:
            # get global addrs set if multiple
            globalAddrs = self.getGlobal()

            if filterByPrefix is None:
                return self.__padIp6Addr(globalAddrs[0])
            else:
                for line in globalAddrs:
                    line = self.__padIp6Addr(line)
                    print "Padded IPv6 Address:" + line
                    if line.startswith(filterByPrefix):
                        return line
                print 'no global address matched'
                return str(globalAddrs[0])
        except Exception, e:
            ModuleHelper.WriteIntoDebugLogger('getGUA() Error: ' + str(e))
            return e