I was having a problem today trying to install NixOS in a VirtualBox guest machine on a macOS host where it would never get past “running udev…”. After a bunch of futzing I finally got it to work so I figured I’d post this in case anyone else has the same issue. Also maybe someone might want to update the instructions at https://nixos.org/nixos/manual/index.html#sec-instaling-virtualbox-guest.
The symptoms were it would go through stage 1 and end up at “running udev…” and never print anything else after that.
What fixed it was going into the virtual machine settings, Display, Screen, and changing “Graphics Controller” to “VBoxVGA”. After doing this, it was able to get through the boot process and bring me to the root login.