Fixes error
Can't use an undefined value as an ARRAY reference at /nix/store/...-install-grub.pl line 642, <FILE> line 5.
with `/boot/grub/state` being:
```
grub
2.04
no
/dev/sda
/boot
```
I am not sure where the trailing empty line can come from; the script does not
seem to write it. In any case, now we handle that situation as well.
Further, ensure that `extraGrubInstallArgs` defaults to the empty array
if its key is not present in the `jsonState`.
For example, turns the error
cannot copy /nix/store/g24xsmmsz46hzi6whv7qwwn17myn3jfq-grub-2.04/share/grub/unicode.pf2 to /boot
into the more useful
cannot copy /nix/store/g24xsmmsz46hzi6whv7qwwn17myn3jfq-grub-2.04/share/grub/unicode.pf2 to /boot: Read-only file system
Useful for when you need to build grub modules into your grub kernel
to get a working boot, as shown in the added example.
To store this new value, we switch to more structural JSON approach.
Using one line per value to store in `/boot/grub/state` gets really messy when
the values are arrays, or even worse, can contain newlines (escaping would be
needed). Further, removing a value from the file would get extra messy
(empty lines we'd have to keep for backwards compatibility).
Thus, from now on we use JSON to store all values we'll need in the future.
Without this, systemd-boot does not add an EFI boot entry for itself.
The reason it worked before this fix is because it would fall back to
the default installed \EFI\BOOT\BOOTX64.EFI
boot.loader.grub.device` was hardcoded to `bootDevice`, which is
wrong, because that's the device for `/`, and with `useBootLoader`
the boot loader is not on that device.
This bug probably came into existence because of bad naming;
`virtualisation.bootDevice` has description
"The disk to be used for the root filesystem", which is very confusing;
it should be `.rootDevice` then!
Unfortunately, the description is right and the attribute name is wrong,
so it is not easy to change this without deprecation.
This commit ensures that even if you use `useBootLoader` and
`diskInterface == "scsi"`, the created VM can boot through, and can run
`nixos-rebuild afterwards.
It also adds extra commentary to explain what's going on in this module
in general in relation to `useBootLoader`.
