'adb command, add -s serial by default. return the subprocess.Popen object.'

def cmd(self, *args, **kwargs):

    serial = self.device_serial()
    if serial:
        if ('    ' in serial):
            serial = ("'%s'" % serial)
        return self.raw_cmd(*(['-s', serial] + list(args)))
    else:
        return self.raw_cmd(*args)
