CA-130371: don't die of footer/header VHD size mismatch

There's no need to kill tapdisk via an assertion failure in this case as
there might be an intermittent storage error.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>

GitHub: closes #71 on xapi-project/blktap