This is definitely possible both with
VirtualBox and any other mainstream virtualization product as well I guess.
VMware vSphere sandbox, I have created an additional virtual switch that is not mapped to any physical network adapters. This network is not visible/reachable from the outside world, but VMs attached to it can reach each other through interfaces using this switch, so it's perfect for a RAC private network with 1 host machine.
VirtualBox, I use
NAT Network (available since version 4.3) for RAC private network. It's a similar concept, a virtual network not visible/reachable from the outside world, but VMs with interfaces in this network can communicate each other through it.
You can use the above for RAC public networks as well, so you don't need to allocate any IP addresses on your real physical network. For example, in
VirtualBox, you can use 1
NAT network interface as "management interface", and define port forwarding rules on it, so you can access the VM from your local machine thorugh RDP/SSH without using the VM console. Then use another
NAT Network interface for the RAC public network, and another
NAT Network interface for the RAC private network, so 3 interfaces in total.
I would not bother with
VirtualBox support shared virtual disks.