In a VirtualBox guest, VBoxService (which is for example responsible for setting the time after a host suspend/resume) is not started on boot. I thought about writing a simple init script to doit (editing the vboxadd init script seemed like a bad idea, since it might get overwritten on updates).
Then I decided to try the “native systemd” way.
Executive summary: it is much easier than with old style SysV init:
# cat /etc/systemd/system/vboxservice.service [Unit] Description=VirtualBox guest services [Service] ExecStart=/usr/bin/VBoxService -f [Install] WantedBy=default.target
systemctl enable vboxservice.service.