Why not just configure an FQDN before installing a control panel? Virtualmin will attempt to set it up for you, but it can sometimes fail on virtual machines, as virtual machines often have weird network configurations that aren't easy to detect reliably with a few lines of shell script. But, you really do want to have a FQDN on your system; you don't need to agonize much about what that FQDN is, as long as it is in a domain you have control over (so you can add DNS records for it).
A fully qualified domain name is one of the form "virtualmin.com" or "srv1.virtualmin.com" or "ns1.virtualmin.com". We recommend you use one of the latter, rather than a bare domain name (e.g. use something like "server.virtualmin.com" instead of "virtualmin.com", because the latter has several side effects and limits our ability to do some things with DNS later).
You haven't said whether you actually entered a domain name for Virtualmin during that installation. As I mentioned, in most cases, if you do that, Virtualmin's install script will set it up for you, and proceed normally. If Virtualmin can't set it up for you, then you'd want to add the name manually to your hosts file for the IP address of the system, and use the hostname command to set it for the current session. This is pretty basic stuff, that you'll need to know going forward...Virtualmin makes loads of things easy and fast, but you still have to understand some of the underlying technology and concepts.
Note: I work on Virtualmin. If you continue to have problems installing Virtualmin, you may want to ask follow up questions on the Virtualmin forums (I don't normally follow things going on here, but I happened to get a Google alert about it). I'm sure this forum also has lots of knowledgeable folks, but the Virtualmin forums have more experienced Virtualmin users on hand, so questions specific to Virtualmin would likely get better answers there. Though this is not really a Virtualmin question; any server will need a FQDN, and any control panel will need it set, in order to correctly manage mail and some DNS functions. If you try to install a control panel and it doesn't require one, then it's a control panel that's probably going to make your life harder and more confusing in the future. (To be clear: don't keep trying new control panels until you find one that doesn't require a FQDN, because that's just pushing the problem out into the future, where it will be much more time-consuming to deal with.)