I work with VirtualBox a lot. Unfortunately, if you want to automatically mount shared folders after boot, it is not so easy. Why? During boot is not network ready so it is necessary to mount a drive after network boot-up. Here is a small trick what to add to fstab to make it work…
/etc/fstab
shared_folder /mount_point vboxsf uid=your_uid,noauto,x-systemd.automount