The mystery gets more interesting... I now have a CentOS 7.3 Dell R710 server doing the exact same thing of rebooting immediately after the Xen kernel load. Just to note this is a second system and not just the first system with an update. I hope I'm not introducing something odd. They only "interesting" thing I have done for historical reasons is to change the following /etc/sysconfig/grub line:
GRUB_CMDLINE_XEN_DEFAULT="dom0_mem=6G,max:8G cpuinfo com1=115200,8n1 console=com1,tty loglvl=all guest_loglvl=all"
But I've done that on other servers without issue. In fact I have a Dell R710 that DOES work with CentOS 7 and the new kernel... so confused.