xvfb fails to start, missing dependencies on xauth and x11-xkb-utils

this breaks several builds using xvfb running testsuites

$ xvfb-run -a -e xvfb-run.log true
$ cat xvfb-run.log

record: RECORD extension enabled at configure time.
record: This extension is known to be broken, disabling extension now..
record: http://bugs.freedesktop.org/show_bug.cgi?id=20500
[ 0.000000] (EE) AIGLX error: dlopen of /usr/lib/dri/swrast_dri.so failed (/usr/lib/dri/swrast_dri.so: cannot open shared object file: No such file or directory)
[ 0.000169] (EE) GLX: could not load software renderer
[dix] Could not init font path element /usr/share/fonts/X11/misc, removing from list!
[dix] Could not init font path element /usr/share/fonts/X11/cyrillic, removing from list!
[dix] Could not init font path element /usr/share/fonts/X11/100dpi/:unscaled, removing from list!
[dix] Could not init font path element /usr/share/fonts/X11/75dpi/:unscaled, removing from list!
[dix] Could not init font path element /usr/share/fonts/X11/Type1, removing from list!
[dix] Could not init font path element /usr/share/fonts/X11/100dpi, removing from list!
[dix] Could not init font path element /usr/share/fonts/X11/75dpi, removing from list!
[dix] Could not init font path element /var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType, removing from list!
sh: /usr/bin/xkbcomp: not found
[ 0.072281] (EE) Error compiling keymap (-)
[ 0.072430] (EE) XKB: Couldn't compile keymap
XKB: Failed to compile keymap
Keyboard initialization failed. This could be a missing or incorrect setup of xkeyboard-config.

Fatal server error:
Failed to activate core devices.

Backtrace:
0: Xvfb (xorg_backtrace+0x18) [0xf7969]
1: Xvfb (xorg_backtrace+0xfff086ba) [0xb]
Segmentation fault at address (nil)

Caught signal 11 (Segmentation fault). Server aborting