I have been trying to run hackintosh in a VirtualBox VM for some iOS compiling. The issue I ran into was that the VM got stuck after booting on a gray screen. After searching around I found a solution that wasn’t really documented anywhere else.
The solution is to enable USB 3.0 for the VM. First I needed to download the Oracle VM VirtualBox Extension Pack. After installing this pack I could choose USB 3.0 (xHCI) controller in the settings of the VM.
After this the VM booted10.14 Mojave as it should.