[HVM][DM] Backport the USB support from QEMU 0.8.1 to the current Xen

[HVM][DM] Backport the USB support from QEMU 0.8.1 to the current Xen
device model. To support USB there are two new optional config lines
that can be added to the guest config file:

    usb=1
        This will enable USB without defining a specific USB device.
        This option is assumed and not needed if the `usbdevice' option
        is given.

    usbdevice='device'
        This will enable USB and also enable support for the given device.
        Currently, the only two devices are `mouse' (a PS/2 mouse) and
        `tablet' (an absolute pointing device).  The advantage of `tablet'
        is that Windows guests will automatically recognize and support this
        device so specifying the config line:

                usbdevice='tablet'

        will create a mouse that works transparently with Windows guests
        under VNC.  (Linux doesn't recognize the USB tablet yet so Linux
        guests under VNC will still need the Summagraphics emulation.)

Signed-off-by: Don Dugger <donald.d.dugger@intel.com>