There are a couple of gotchas to running Virt-Manager with Zentyal.
1)You need to connect via ssh.
2) you need super user privileges to inter react with the VMs.
This means you will end up adding a password to the root account which is not best practice.
It has been awhile but as I recall, you can either use Virt-manager to provision and manage VMs or the Zentyal interface. You can't mix and match.