It seems like Debian 8 VMs include MBR, not GRUB. When you see 1FA, it shows a list of options but isn't as powerful as GRUB.
Why exactly do you need to access the boot menu? If you need to select a different disk, press "a" and then the number of partition.
If your issue is that you lost your root password, unfortunately there isn't much you can do with MBR. You could try to boot a LiveCD, execute "chroot /path/to/disk" and changing the root user credentials with "sudo passwd".