Ditto! I was having this problem too while installing arch on Virtualbox. It was fixed after I did the following.
1) Execute the following to install "mesa" and virtualbox support:
pacman -S mesa
pacman -S virtualbox-guest-utils
2) Open "/etc/modules-load.d/virtualbox.conf" with nano.
and write the following lines in the file
press Ctrl + X , enter Y to confirm that you want to "save and exit" and when it asks for the filename hit return, without changing the name.
3) Reboot by typing.
4) Install the following packages, if you have not already. (I had them installed before, so I skipped this part)
pacman -S xorg-twm xorg-xclock xterm
5) Then, just type:
That's it. Hope it helps! ^_^