A USB keyboard may return key codes in any of the 6 key code array
positions (issue #48). So we have to compare the latest report to
the previous one to detect new key presses and handle rollover. As
the same change is needed for all HCI types, factor out the common
code into the base driver.
When an interrupt transfer completes, the UHCI loads the QELP in the
QH with the link pointer from the TD (which is a null pointer in this
case). We need to set the QELP back to point at the TD to enable the
next interrupt transfer.
This gets us to the point that a keyboard is detected when running
QEMU with a EHCI/UHCI combination controller and the first keypress
is detected and returned.
