Installation is processed as root (sudoer) and consequently enabling
"manually" (by creating link) xds-agent service during install is not
a good idea. The only viable solution is to well document how user
should enable it as a user systemd service, IOW:
systemctl --user enable xds-agent.service
Change-Id: Iba1f3bb501c099606b53d6cf39239af499787aa7
Signed-off-by: Sebastien Douheret <sebastien.douheret@iot.bzh>