First, you have to generate a private / public key pair and publish your public key on the host server running the virtual machines.
Information about it can easily be found on the web.
After, simply write your private key in the file id_rsa in the .ssh folder located in your home folder.
If the folder .ssh doesn't exist you can create it.
Ensure that only your user has the right to read that id_rsa file, or the key will be ignored when connecting to ssh :
> chmod 600 id_rsaIn the Add Connection form of the Virtual Machine Manager, select SSH method and root username (or any other user on the remote server where is published your public key)