The sleep/wake button on the right hand side of the device now triggers Siri when you hold it down for long enough.
You can now reboot from Settings in iOS11 if the device is responsive and unlocked.
If it's locked or you just want to properly shutdown quickly, press and hold a volume button and the lock button to bring up the SOS mode, which includes the shut down slider.
If the device is unresponsive, it's much less obvious now: https://ios.gadgethacks.com/how-to/force-restart-iphone-x-wh... Volume up, then down, then press and hold the lock button.