qdev: Change Property::offset field to ptrdiff_t type

Property::offset field is calculated as a diff between two pointers:

  arrayprop->prop.offset = eltptr - (void *)dev;

If offset is declared as int, this subtraction can cause type overflow,
thus leading to failure of the subsequent assertion:

  assert(qdev_get_prop_ptr(dev, &arrayprop->prop) == eltptr);

So ptrdiff_t should be used instead.

Signed-off-by: Ildar Isaev <ild@inbox.ru>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
