guests/linux: Emit upstart event `vagrant-mounted` also on NFS mounts

Complements #2502, and fixes error on VMs without upstart (like #2596).
guests/linux: Emit upstart event on VirtualBox using the expanded path
Merge pull request #2705 from tmatilai/upstart-emit-nfs

Emit upstart event `vagrant-mounted` also on NFS mounts