Xorg on archlinux, it need to be configured. One way to do it is to run:
Xorg :0 -configure cp ~/xorg.conf.new /etc/X11/xorg.conf
The problem may be related to https://www.virtualbox.org/ticket/14732. In such a case you would need to use the newest VirtualBox (5.0.18), and also install guest additions before starting X.