Partition saving doesn't notice MBR partitions

[bgilbert]

Bug

Host Operating System Version

Any

Target Operating System Version

Any

coreos-installer Version

All releases through 0.13.1

Expected Behavior

coreos-installer install --save-partindex correctly saves partitions on disks that have an MBR partition table and no GPT.

Actual Behavior

coreos-installer install --save-partindex clobbers such partitions.

Reproduction Steps

1. Use fdisk to create a partition on a blank target disk
2. Install to the disk, passing --save-partindex to save the partition

Other Information

--save-partlabel isn't affected because MBR doesn't support partition labels.

We could support this via mbrman. We'd need to detect and ignore protective MBRs, and to fail if the MBR otherwise doesn't match a valid GPT.

[prestist]

In speaking with @bgilbert we are going to be taking this issue with a 2 pronged approach

1. Mitigation reduce risk of data loss when there is an MBR with partitions we should save.
2. Migrate MBR saved partitions to GPT format so the installer can effectively save them.

Keeping in mind, that a disagreeing MBR/GPT shouldn't happen; however, its possible, and the feature is intended for preserving data from pre-existing disks with an unknown history.

That being said, this issue will become the stage 1.
Stage two will be linked here and no longer considered a part of this issue.