Hi,
I just downgraded back to 4.3.10 and it's working again, but 4.3.12 kept complaining about screens (e.g. "number of created screens does not match number of detected devices")
My configuration:
Host MacBook Pro Retina with OS X 10.9.2
VirtualBox: 4.3.12 with VirtualBox Extension Pack
Guest: Linux Mint 15 "Olivia" kernel 3.8.0 32bit using xfce running updated Guest Additions
After upgrading VirtulBox and Guest Additions the system booted in text mode and I couldn't find a way to fix the problem. Downgrading worked.