MT#6711 Solve shared folders issue by create symlink for mount.vboxsf

We're still facing:

| root@sp1:~# mount -t vboxsf -o uid=`id -u sipwise`,gid=`getent group sipwise | cut -d: -f3` /vagrant /vagrant
| mount: Protocol error

on some VMs (noticed on e.g. 3.1 PRO system with 3.13-0.bpo.1-amd64).
The /sbin/mount.vboxsf file is missing for some reason we're not
aware of yet.

deployment.sh

@@ -1866,9 +1866,15 @@ vagrant_configuration() {
 
   # work around regression in virtualbox-guest-additions-iso 4.3.10
   if [ -d ${TARGET}/opt/VBoxGuestAdditions-4.3.10 ] ; then
+    echo "Installing VBoxGuestAddition symlink to work around vbox 4.3.10 issue"
     ln -s /opt/VBoxGuestAdditions-4.3.10/lib/VBoxGuestAdditions ${TARGET}/usr/lib/VBoxGuestAdditions
   fi
 
+  if ! [ -e ${TARGET}/sbin/mount.vboxsf ] && [ -e ${TARGET}/usr/lib/x86_64-linux-gnu/VBoxGuestAdditions/mount.vboxsf ] ; then
+    echo "Installing mount.vboxsf symlink to work around vbox issue"
+    ln -s /usr/lib/x86_64-linux-gnu/VBoxGuestAdditions/mount.vboxsf ${TARGET}/sbin/mount.vboxsf
+  fi
+
   # MACs are different on buildbox and on local VirtualBox
   # see http://ablecoder.com/b/2012/04/09/vagrant-broken-networking-when-packaging-ubuntu-boxes/
   echo "Removing /etc/udev/rules.d/70-persistent-net.rules"