$ dpkg --configure virtualbox-guest-x11
Setting up virtualbox-guest-x11 (4.1.12-dfsg-2ubuntu0.6) ...

Configuration file `/etc/X11/Xsession.d/98vboxadd-xclient'
 ==> File on system created by you or by a script.
 ==> File also in package provided by package maintainer.
   What would you like to do about it ?  Your options are:
    Y or I  : install the package maintainer's version
    N or O  : keep your currently-installed version
      D     : show the differences between the versions
      Z     : start a shell to examine the situation
 The default action is to keep your current version.
*** 98vboxadd-xclient (Y/I/N/O/D/Z) [default=N] ?

yes | dpkg --configure virtualbox-guest-x11

echo y | dpkg --configure virtualbox-guest-x11