win-usb-dev: Set UsbDk backend on libusb 1.0.22

This patch configures libusb in the context used in win-usb-dev, with
the same backend as the context initialized in usb-device-manager.

Otherwise spice_usb_device_manager_libdev_match returns false for all
devices, and no usb device is offered to end user for redirection.

Acked-by: Christophe Fergeau <cfergeau@redhat.com>