This port of libusb supports a single USB device, which is requested from the
USB host driver via a USB session labeled 'usb_device'. The assignment of an
actual USB device to the application must be defined as a 'policy' in the USB
host driver's configuration as described in the USB driver's README file.

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 the libusb VFS plugin
(vfs_libusb.lib.so) present in its VFS:

! + vfs
!   + dir dev
!     + libusb
