I have yet to successfully reboot a host from XO. I have tools installed on three out of four VM. I try shutting down the VM without tools, then issue reboot host in XO GUI. Get warning:
Some VMs cannot be migrated without first rebooting this host. Please try force reboot.

Is this a setting perhaps?

I'm always needing to shutdown all VMs before I can reboot the host. It is a single host environment, so there will never be an available server to migrate VMs to.

Rebooting in XSCONSOLE causes the host to be unavailable in XO. I have to remove then add the host back in XO.

Can anyone shed light as to what I'm doing wrong?

Do I just need to always run the following after a reboot?
xe host-enable host=xxxx