In Xen>=4.3, libxl supports new syntax for USB devices:
usbdevice=[ "DEVICE", "DEVICE", ... ]
Add support for that in xenconfig driver. When only one device is
defined, keep using old syntax for backward compatibility.

Adjust tests for changed options order.
Signed-off-by: NMarek Marczykowski-Górecki <marmarek@invisiblethingslab.com>