I am using Xen 4.6.0-8.el7 on CentOS 7 with 3 Linux DomUs. I did have Xen 4.? on CentOS 6.5 on my previous system. I installed using  yum install centos-release-xen.
On shutting down the Hypervisor it used to wait for the DomUs to shut down gracefully (one can take a minute or more), then on starting up again it would restart the DomUs, all nice and controlled.
For some reason the DomUs (CentOS or Ubuntu servers) are not shutting down nicely when the Hypervisor is shutdown, and not restarting. I have the cfg files in /etc/xen/auto so they should start up.

If I forget to shut down the DomU guests then the shutdown hangs.

I am running this on a Dell Poweredge T430 by the way.
Any ideas why, and how to cure this?