vmstate-static-checker: fix size mismatch detection in unused fields

If a field changed from something to unused, the checker wasn't flagging
if the field size mismatched.  This was noticed in:

http://thread.gmane.org/gmane.comp.emulators.qemu/419802

where the 4->1 size change along with field name change to 'unused'
wasn't being flagged.  Fix this.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Message-Id: <d7ec03a9b2edfa0616764887a51ba8f64fdd3f68.1466165736.git.amit.shah@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
