qcow-efi backend ignores `diskSize`

[ereslibre]

Create a qcow-efi image with ./nixos-generate -c configuration.nix -f qcow-efi --disk-size 20480 on the root of this repository.

The generated qcow image has a different disk size than specified:

/dev/disk/by-label/nixos  2.9G  1.5G  1.2G  55% /

It effectively defaults to the qemu-efi module option of boot.diskSize = "auto";, given nothing sets the disk size as the module option expects (boot.diskSize).

[ereslibre]

With #368 applied, the same command results in:

/dev/disk/by-label/nixos   20G  1.7G   17G  10% /