How can I disable power saving for all devices in Device Manager?

The power options are not in the device manager. To access them, go to your control panel, Hardware & Sound, Power Options, click on the left panel option to Create Power Plan and select High Performance.

When you get back to the Power Options screen, click Change Plan Settings next to your new plan. You'll probably want to select the Hard disk section to never turn it off and also the Sleep option to disable hibernation.

Open Device manager or type in devmgmt.msc in the search or Run box.

Now you want to expand the Universal Serial Bus Controllers

Make sure you choose USB Root Hub do this to both.

Uncheck the box on both and Hit OK

This will solve your issue. Now anything you plug into your usb ports will not be turned off to save power.

EDIT:

This will be the best way to do it. Explained below.

I wish to turn off this option for all my devices, including new devices that may appear in the system.

You want to turn this off for all devices including new devices that may appear in the system. Turning this off for your USB devices as explained above will turn it off or any device plugged in via usb.

Doing this to all the individual Devices that you do not want the system to turn off to save power. Is better then changing your whole systems power performance which will cause reduced battery time.

I understand you want this done for all devices. System ones will most likely not shut themselves off unless they are set to. If you have bluetooth,RFID devices just un check the "Allow the computer to turn off this device to save power"