libxl: spice usbredirection support for upstream qemu

libxl: spice usbredirection support for upstream qemu

Usage: spiceusbredirection=NUMBER (default=0)

Enables spice usbredirection. Creates NUMBER usbredirection channels
for redirection of up to 4 usb devices from spice client to domU's qemu.
It requires an usb controller and if not defined will automatically adds
an usb2 controller.

Changes from v3:
- fixed condition that enable usbversion if it isn't defined in presence
  of usbredirection enabled

Changes from v2:
- updated for usbversion patch v7
- now usbredirection cannot be used with usb and usbdevice parameters
- if usbversion is undefined it will creates an usb2 controller

Changes from v1:
- Now can be setted the number of redirection channels.
- Various code improvements.

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>