Add a backend for para-virtualized USB devices for xen domains.

The backend is using host-libusb to forward USB requests from a
domain via libusb to the real device(s) passed through.
Signed-off-by: NJuergen Gross <jgross@suse.com>
Acked-by: NAnthony PERARD <anthony.perard@citrix.com>
Message-id: 1463062421-613-4-git-send-email-jgross@suse.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>