SCTX-1536: Option to use the back-up footer instead of the primary one.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Keith Petley <keith.petley@citrix.com>

GitHub: closes #55 on xapi-project/blktap
CA-116436: Don't access pre-allocated but unwritten BAT entries

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Keith Petley <keith.petley@citrix.com>

GitHub: closes #57 on xapi-project/blktap