CA-126139: additional debugging

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>
CA-126139: don't check for non-NULL-terminated XenStore values

According to the documentation of xs_read the string returned contains
the NULL-terminating character, so we replace the handling of that
scenario with a simple sanity check.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>
Merge CA-126139

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #72 on xapi-project/blktap