Fix virtualbox problems running Ubuntu guest

Problem: after upgrading the Ubuntu guest (running on Windows 7 host) to 15.10, 
1. The full screen mode stopped working; 
2. The shared folder with windows host stopped working; 

Fixes: 
1. Upgraded to virtual box version 5.0.10 (the latest version at the time). 
2. Install VirtualBox Guest Additional as usual 
  # sudo mount /dev/sr0 /media/cdrom
  # cd /media/cdrom
  # sudo ./VBoxLinuxAddtionRun
  But failed to install Windows system drivers because of "unknown X11 version"
  This is fixed by "sudo apt-get install virtualbox-guest-x11" package. 
  After a reboot, the full screen comes back. 
3. Turn out there is a bug in virtualbox software, it did not update the symbolic link /sbin/mount.vboxsf points to, 
   and it still points to the old one which was cleaned during step 2. 
   # sudo rm /sbin/mount.vboxsf 
   # sudo ln -s /opt/VBoxGuestAdditions-5.0.10/lib/VBoxGuestAdditions/mount.vboxsf /sbin/mount.vboxsf 
  with a reboot, the shared folder now works.