Hi,
I think that the steps to achieve your goal are:
1. Copy the VMs disks from your old Zentyal server to the new one.
2. Create the virtual machines with the same configuration but don't configure anything in the tab 'Device Settings' yet.
3. Once the directory of each VM have been created, move the hard drives to the correct directory.
4. Edit the VMs and configure the hard drive of that VM in the 'Device Settings' tab.
5. Finally, save changes and start the VM.
I don't know what is your current environment and also I didn't test this solution but I think it should work.