libxl: u.hvm.usbdevice_list is checked for emptiness

libxl: u.hvm.usbdevice_list is checked for emptiness

Currently usbdevice_list is only checked for nullity. But the OCaml
binding will convert empty list to a pointer to NULL, instead of a
NULL pointer. That means the OCaml binding will fail to disable USB.

This patch will check emptiness of usbdevice_list. And NULL is still a
valid empty list.

Signed-off-by: Robin Lee <robinlee.sysu@gmail.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>