My bad, it should be grub.cfg
instead of grubx64.cfg
.
Full steps from the beginning:
Copy files from /boot/efi/EFI/qubes/ to /boot/efi/EFI/BOOT/:
sudo cp -r /boot/efi/EFI/qubes/* /boot/efi/EFI/BOOT/.
Change filenames grubx64.efi → BOOTX64.efi and grub.cfg → BOOTX64.cfg:
sudo mv /boot/efi/EFI/BOOT/grubx64.efi /boot/efi/EFI/BOOT/BOOTX64.efi
sudo mv /boot/efi/EFI/BOOT/grub.cfg /boot/efi/EFI/BOOT/BOOTX64.cfg
This should be run from dom0 in Qubes OS.