rbd: use JNA LongByReference type to marshall C ssize_t type

@wido, I replaced IntByReference with LongByReference in image listing and snap children listing functions. The reason behind the change is LongByReference maps more closely with ssize_t which is used by the underlying librbd functions (rbd_list and rbd_list_children). In addition, using IntByReference also throws off the librbd logic and results in a seg fault when the input byte buffer to rbd_list() is smaller than the required size.