Isso pode ser um problema com a instalação do pacote para o 14.0.4. No 16.0.4:
$sudo apt-get install xvfb
$Xvfb :99 -screen 0 1024x768x24 > /dev/null &
$ xdpyinfo -display :99 | grep "number of extensions" -A 25
number of extensions: 25
BIG-REQUESTS
Composite
DAMAGE
DOUBLE-BUFFER
DPMS
GLX
Generic Event Extension
MIT-SCREEN-SAVER
MIT-SHM
Present
**** RANDR ****
RECORD
RENDER
SECURITY
SGI-GLX
SHAPE
SYNC
X-Resource
XC-MISC
XFIXES
XINERAMA
XInputExtension
XKEYBOARD
XTEST
XVideo