CA-148791: don't use absolute path for list_domains

list_domains used to live in /opt/xensource/bin in Creedence and in
/usr/bin in trunk, and since there's no good reason for using absolute
paths, so let's drop absolute paths.

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

GitHub: closes #231 on xapi-project/sm