emulators/virtualbox-ose*: Add support of "Unattended Installation of Guest OS" feature
Install files required by Unattended Installation of Guest OS feature.
These files were already prepared during the ports' build, so just add
them to the installation.
Details about this feature are here:
https://www.virtualbox.org/manual/topics/create-vm.html#tk_create-vm-unattended-install
Reported by: xin3qu via IRC (#freebsd-vbox @ Libera.Chat)