CA-135194: use better names for XenStore watch tokens

helps debugging

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

GitHub: closes #82 on xapi-project/blktap
CA-135194: fix resource clean up in error path

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

GitHub: closes #82 on xapi-project/blktap