I see what you meant, thanks. The culprit is that the USB core is now built as a module where it was built into kernel in the earlier distro versions. You need to pass usbcore.nousb=1 instead. IMO, this is a bug in the code, though. The driver should refer to nousb no matter how it's built. Also, xhci_pci is still loaded even if usb is disabled. This is another thing to be fixed. Other host drivers refuse to load.