xhci: Initial xHCI implementation

Based on the implementation from Hector Martin <hector@marcansoft.com>

Hectors's implementation completely sidestepped the qemu usb system and
used libusb directly for usb device pass through.  So I've ripped out
the libusb bits (or left them in disabled, as reference for further
coding) and hooked up the qemu subsystem instead.  That work is not
complete yet though, partly due to limitations of the qemu usb
subsystem.  Nevertheless I think it is better to continue development
in-tree, especially as the qemu usb bits need a bunch of improvements
too for decent usb 3.0 support.

Current state:
  - usb-storage emulation should work ok.
  - Devices which need constant polling (HID emulation like usb-tablet)
    are known to not work.
  - ISO xfers are not implemented yet.
  - superspeed ports are not implemented yet.
  - usb pass-through is completely untested so far.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
