Refuse to evaluate if zpool is set to mirror if only one device is given

[cheriimoya]

When only giving one disk, installation fails during runtime with nixos-anywhere.

Minimal config:

{
  disko.devices = {
    disk = {
      hdd1 = {
        type = "disk";
        device = "/dev/sda";
        content = {
          type = "zfs";
          pool = "zroot";
        };
      };
    };
    zpool = {
      zroot = {
        type = "zpool";
        mode = "mirror";
        mountpoint = "/";
      };
    };
  };
}

Output:

+ zpool create -f zroot -R /mnt mirror /dev/sda
invalid vdev specification: mirror requires at least 2 devices

Probably this is also a problem with raidz, raidz2 and raidz3 where one could check for at least X disks.