Bug#911810: debian-installer: cannot boot installed on EFI with single xfs root

[Yangfl <mmyangfl@gmail.com>]

Package: debian-installer
Severity: important

I found with the lastest weekly iso (found in
debian-cdimage/weekly-builds/amd64/iso-cd/debian-testing-amd64-netinst.iso)
installing with xfs+uefi lead to a non-bootable system due to grub not
being able to find the xfs module, without any notice or warning
during setup. I've confirmed it on KVM+OVMF / Hyper-V gen 2.

Step to reproduce:

 1. setup VM with EFI enabled
 2. when partitioning, use xfs as root (/) fs, no /boot, and a EFI system
    partition (if you omit EFI system partition, setup won't let you continue)
 3. finish installation, not bootable, only a plain GRUB prompt
 4. grub> insmod xfs
    error: file '/boot/grub/x86_64-efi/xfs.mod' not found.
    grub> set root=(hd0,`Tab`
    Possible partitions are:

      Partition hd0,gpt1: Filesystem type fat, UUID xxxxx
      Partition hd0,gpt2: No known filesystem detected

It doesn't happen with a separate /boot partition (in ext4) present.