This setup only works if your VM is not using Undo Disks.

To configure the auto-restart of virtual machines after a reboot of the host machine, you need to first stop the VM (configuration can only be changed while it is in the "Off" state. 

Once the VM has been stopped, go to Configuration > General properties:
1) Click checkbox for "Run virtual machine under the following user account."
2) Enter user credentials (if you are using domain credentials, don’t forget to put DOMAINusername).
3) Action dropdown – select "Automatically turn on virtual machine if it was running when Virtual Server stopped.
4) Press Ok and you should be all set.