Code review comment for ~raharper/curtin:fix/block-sfdisk-parsing

> Dan Watkins (daniel-thewatkins) wrote on 2020-05-01:
> `--json` was introduced in util-linux 2.27[0] which means it isn't present pre-xenial[1] or in Centos 7[2]. Is that an issue?
>
>
> [0] https://github.com/karelzak/util-linux/blob/master/Documentation/releases/v2.27-ReleaseNotes#L611
> [1] https://launchpad.net/ubuntu/+source/util-linux
> [2] Assuming that the version listed on http://mirror.centos.org/centos/7/os/x86_64/Packages/ is the version in Centos 7, that is.

At least for now; it is not an issue. Subiquity is the only user of the
'preserve' flag; MAAS does not have such use-case; and for now the
partition_verify() which utilizes this code-path is only triggered for a
preserve scenario (which is also Ubuntu only).

Alternatively, we could also fix the existing code and use it as a fallback if
--json output failed.