nixos/tests/installer: add bpool for grub files in zfsroot test

    createPartitions = ''

      machine.succeed(

          "flock /dev/vda parted --script /dev/vda -- mklabel msdos"

          + " mkpart primary linux-swap 1M 1024M"

          + " mkpart primary 1024M -1s",

          + " mkpart primary 1M 100MB"  # bpool

          + " mkpart primary linux-swap 100M 1024M"

          + " mkpart primary 1024M -1s", # rpool

          "udevadm settle",

          "mkswap /dev/vda1 -L swap",

          "mkswap /dev/vda2 -L swap",

          "swapon -L swap",

          "zpool create rpool /dev/vda2",

          "zpool create rpool /dev/vda3",

          "zfs create -o mountpoint=legacy rpool/root",

          "mount -t zfs rpool/root /mnt",

          "zfs create -o mountpoint=legacy rpool/root/usr",

          "mkdir /mnt/usr",

          "mount -t zfs rpool/root/usr /mnt/usr",

          "zpool create -o compatibility=grub2 bpool /dev/vda1",

          "zfs create -o mountpoint=legacy bpool/boot",

          "mkdir /mnt/boot",

          "mount -t zfs bpool/boot /mnt/boot",

          "udevadm settle",

      )

    '';

    # umount & export bpool before shutdown

    # this is a fix for "cannot import 'bpool': pool was previously in use from another system."

    postInstallCommands = ''

      machine.succeed("umount /mnt/boot")

      machine.succeed("zpool export bpool")

    '';

  };

  # Create two physical LVM partitions combined into one volume group