sgdisk Exit code: 4 when create partitions on md RAID1 on ppc64le machine

[Aliaksandr Vasiuk]

Hi,

We are using MAAS 3.2.10.
I will attach screenshots of the storage layout and the curtin install.log to the report.
When deploying ppc64le IBM 8335-GTH machine with a md software RAID1 curtin fails to create partitions with
```
        Running command ['sgdisk', '--new', '2:1852794880:1950443519', '--typecode=2:8300', '/dev/md0'] with allowed return codes [0] (capture=True)
        An error occured handling 'md0-part2': ProcessExecutionError - Unexpected error while running command.
        Command: ['sgdisk', '--new', '2:1852794880:1950443519', '--typecode=2:8300', '/dev/md0']
        Exit code: 4
```
Exit code 4 of sgdisk seems to suggest `Non-GPT disk detected and no -g option`.

The workaround is to create several md RAIDs instead of partitioning a single RAID.

Best Regards,
Alex.