Currently, libusb supports a single USB device, which it requests from the USB
driver with the 'usb_device' label.

The policy of the USB driver configures the actual device to be provided to
the application using libusb:

<config>
	<raw>
		<policy label="application -> usb_device" vendor_id="0xXXXX" product_id="0xXXXX"/>
	</raw>
</config>

See also the README file of the USB driver for additional policy attributes.

The Genode USB connection object resides in a VFS plugin named 'vfs_libusb',
which is necessary to handle the 'ack_avail' signal in the libc kernel context.
So, an application using libusb needs to have a '<dir name="dev"><libusb/></dev>'
node in its VFS configuration and the 'vfs_libusb.lib.so' file loadable at
runtime.
