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>
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