A maybe longwinded resolution but one that works for me
I had the same problem when first I got as far as the grub config back in 7.x
My setup is on virtual box as well and uses separate HD for LFS and the build system.
I installed the LFS bootloader to the LFS HD not the hostOS HD, so pressing f12 during boot allows me to choose what HD the system boots from; the hostOS system or LFS.
If you have LFS on a separate partition on the same drive as your hostOS make a new HD image, live-boot an ubuntu iso in your virtual machine and use gparted to clone the LFS partition to the new drive, then install
https://help.ubuntu.com/community/Boot-Repair and recover the original system boot.
You would have to set up the linked folders across the HD's shouldn't be too bad, or use this script as a template
https://pastebin.com/hbzvxueM . (its a copy and paste out of the 9.0 LFS handbook with a few changes)
I hope you find this useful