Finish xHCI USB Driver for Modern Hardware

Hi!

I noticed there appears to be an unfinished xHCI USB driver in the source tree.

Many modern desktop and laptop motherboards provide only xHCI-controlled USB ports, so without xHCI support it's not possible to use a USB keyboard or mouse after booting on this hardware.

It would be great to see the existing xHCI work completed if it's still planned. I'd be happy to test experimental builds on real hardware if that would be helpful.

Thanks for all your work on toaruOS!