Use Vmware VGA driver on Proxmox for KVM Windows Guests

You might well be better off creating it as a full-stack install including the OS and all dependencies, on an OS that you know it works well under. Then let the end user decide if they want to install it standalone or as a VM guest on KVM, VMWare, Xen, or whatever. The only thing you'd gain over this with a KVM image is some amount of hardware-agnosticity, but there are still potential pitfalls there as you've pointed out, and the site would have to be willing and able to maintain a KVM host in addition to your application and its OS.

EDIT: To respond to your reformulated question, I think you've still go to leave this stuff to the sites -- VM guests, by design, cannot control how networking is set up on the host. You can certainly provide documentation and instructions for doing this in the simplest or most effective way, but that may vary from site to site. No matter what you do, if you distribute this as a VM image or appliance, the site will have to effectively set up and maintain a VM host that is specifically configured to work on their network.

After some discussion on the oftc-channel #virt, @laine pointed me to the solution.

Apparently, the resolv.conf file is missing on ubuntu 12.04 (at least on my installation of linux mint 13). It is replaced by some resolvconf directory tree.

To enable internet on the guests: I had to

1. sudo iptables --flush => on the host, might be obsolete, I don't know that. It cleared some rules.
2. add "nameserver8.8.8.8" to /etc/resolv.conf
3. reboot

After that, it worked

EDIT: note that step 2 was also performed on the host. I think it was linux mint (ubuntu 12.04), which was missing the /etc/resolv.conf file.