ppc64: installation may not be booted

the ppc64 systems that we have typically are multipath and have multiple scsi disks.

The order the disks come up in is non-determinable.

curtin has very dumb logic for picking installation device. it basically picks 'sda' by name in this case.
That means that subsequent curtin installs to the same system might instll to different devices, and thus 2 bootable root disks are present.

On power-nv (no virt), the bootloader that is used is petitboot.
On under kvm, the bootloader used is grub via PReP .

petitboot is a linux kernel/initramfs that basically goes looking for grub.cfg files.
if it finds multiple disks with valid grub.cfg, the selection is somewhat non-determinable.

The easiest solution that I have for this is to just wipe the other disks before install (in an early command).
What I have is this in /etc/maas/preseeds/curtin_userdata

early_commands:
  wipestuff: ["sh", "-c", "for d in /dev/sd?; do echo ===wiping $d===; wipefs --all $d || echo failed with $?; done"]