However, when you want to reach the network if the Host OS, you'll have to set the Network Setting to Bridged Adapter.
Open VirtualBox --> right click on the image --> Settings --> Network -->
set "Attached to" to Bridged Adapter.
I only investigated afterwards, but I managed to reach the Host's http://localhost/ from the Guest by connecting to the Ethernet IPv4 address of the host.
On the windows host, open the command line, and run
ipconfighere you'll find your Ethernet IPv4 address. Run something on localhost, and you should now reach the site from the Guest on this ip address.
Read more on VirtualBox networking modes.