SCTX-1536: Don't trust the footer

Don't use the primary VHD footer for file-based VHD VDIs that are
attached in R/W mode because it has been erased. Use the back-up one
instead.

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

GitHub: closes #35 on xapi-project/sm