I'm still getting used to the systemd paradigm on RHEL/CentOS 7 and instinctively reached for chkconfig to add xendomains to the runlevels:
chkconfig --level 2345 xendomains on
This works with older versions of Xen that come with SysV init scripts; later versions are shipping systemd init scripts and must be enabled the systemd way:
systemctl enable xendomains.service
To get a list of systemd scripts:
systemctl list-unit-files