funziona il parsing di piu' elementi, da aggiustare tutto il resto
pusb_conf_parse_options: add one opt_list record for each device.
pusb_conf_device_get_property: use pusb_xpath_get_string_array, print
found devices, then call pusb_conf_parse_options.
pusb_device_connected_array added to this file to iterate over all the
detected devices.
xpath.c: pusb_xpath_get_string_array returns the number of devices found