Comment 13 for bug 1746579

You can add this line in the /etc/default/grub:
GRUB_CMDLINE_LINUX_DEFAULT="break=init"

$ sudo update-grub

But after that you'll have to remove this line manually from initramfs environment:
(initramfs) mount -o rw,remount /root
(initramfs) chroot /root bash
# vi /etc/default/grub
# update-grub

Or, just do a snapshot with virt-manager beforehand.