The VirtualBox online-manual writes “Forwarding host ports < 1024 impossible…”. This tutorial shows that it is still possible. I know that there are several reasons not to do so, but there are also reasons where it is necessary.
VirtualBox with VM configured (not running)
You know the root password
Example for Ubuntu
# become root
$ sudo su -
# start VirtualBox Manager
Now you probably need to import the VM. Attention! The acces rights will be changed!
Configure or just check the port forwarding. To do so, you can use the graphical port forwarding editor or even the command line (VBoxManage).