You could set up a host-only network within Virtualbox. This is a virtual LAN which includes the host and any guests which are attached to it. You would then add a network adapter to each of your guests and attach the adapter to the host-only network. The guests and the host would be able to communicate through the private network.
Once you've done this, processes running on the host could bind specifically to the IP address for the host-only network interface. Only processes running on the host or one of the VMs would be able to contact the host's host-only network address.
The basic process is as follows:
- Start Virtualbox manager and go to File->Preferences->Network->Host-only networks.
- Click the "+" icon at right to create a new one.
- Click the screwdriver icon to access properties for the new network.
- Enable a DHCP server if desired (if you don't do this, you'll have to assign static IPs to each guest).