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: NIldar Isaev <ild@inbox.ru>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

include/hw/qdev-core.h

@@ -237,7 +237,7 @@ struct BusState {
 struct Property {
     const char   *name;
     PropertyInfo *info;
-    int          offset;
+    ptrdiff_t    offset;
     uint8_t      bitnr;
     qtype_code   qtype;
     int64_t      defval;