CA-130898: use 32-bit protocol with guest if omitted

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Acked-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>

(cherry picked from commit 6c86a979700ca4ff1e16fdcc44b3ff2ea4baddda)

GitHub: closes #84 on xapi-project/blktap