qxl: Fix SPICE_RING_PROD_ITEM(), SPICE_RING_CONS_ITEM() sanity check

The pointer arithmetic there is safe, but ugly.  Coverity grouses
about it.  However, the actual comparison is off by one: <= end
instead of < end.  Fix by rewriting the check in a cleaner way.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>