Rather than ditching your 18.04 installation for a 16.04 one, or running 16.04 in a virtual machine, you might want to consider running 16.04 in an LXD/LXC container.
$ sudo snap install lxd
...
$ sudo lxd init
...
$ sudo lxc launch ubuntu:16.04 devbox -c security.nesting=true
...
$ sudo lxc shell devbox
That will install the lxd snap, initialize lxd for use, launch a privileged lxc container named devbox based on a 16.04 image and finally drop you in a shell in the lxc container.
