Hello Kris,
A very short answer (I go to work) from here: Trying to setup Virtualbox for NethServer VM
It was for VirtualBox but the principle is the same for any VM.
Maybe it helps you.
"If you want to use the new VM as Firewall, after first login, you must add and set a new Green interface; after that, you must change eth0 from Green to Red (you cannot change from Green to Red before add a new Green interface).
Attention: you must set additional interfaces in VM VirtualBox if you want to build a Virtual Network. In VM VirtualBox, the additional interfaces will be attached to Internal Network. These interfaces will be used in NS VM for Orange, Blue, Green."