Add workaround for mount folders in Linux guest

See https://github.com/mitchellh/vagrant/issues/3341

This should fix a known issue of VirtualBox 4.3.10

    Failed to mount folders in Linux guest. This is usually because
    the "vboxsf" file system is not available. Please verify that
    the guest additions are properly installed in the guest and
    can work properly. The command attempted was:
    
    mount -t vboxsf -o uid=`id -u vagrant`,gid=`getent group vagrant | cut -d: -f3` /vagrant /vagrant
    mount -t vboxsf -o uid=`id -u vagrant`,gid=`id -g vagrant` /vagrant /vagrant

Signed-off-by: Gianpaolo Macario <gmacario@gmail.com>