libxl_usb: usbctrl, make use of generic device handling functions

libxl_usb: usbctrl, make use of generic device handling functions

Two functions in generate `libxl_device_usbctrl' can be replaced by
generic macro:
- libxl_device_usbctrl_list -> LIBXL_DEFINE_DEVICE_LIST
- libxl_devid_to_device_usbctrl -> LIBXL_DEFINE_DEVID_TO_DEVICE

This patch only needs to define `libxl__usbctrl_devtype.from_xenstore'
to makes use of them.

Small change, libxl_devid_to_device_usbctrl doesn't list all usbctrl
anymore before finding the right one.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>