This is the only way to assign devices rather than fixed gpt partitions.
Without reading the code it's not very obvious how disko actually
assigns devices to zpools.
A config like
```nix
{
vdev = [
{
mode = "mirror";
members = [ "data1" "data2" ];
}
{
members = [ "data3" ];
}
];
}
```
would result in the following command:
```shell
zpool create -f <name> mirror /dev/data1 /dev/data2 /dev/data3
```
which would result in a single vdev with a 3-way mirror, rather than a
vdev with a 2-way mirror and a second vdev with a single disk. By
reordering the vdevs to handle those with an empty mode first we
transform this into:
```shell
zpool create -f <name> /dev/data3 mirror /dev/data1 /dev/data2
```
which does have the desired outcome.
