This action moves second(backup) GPT header to the end of the disk.
It is usable in in-place image expanding, since free space after
second GPT header is unusable. To use additional space, we have
to move second header. This is what sgdisk -e does.
However, sgdisk -e may perform additional actions if the partition
table has unexpected params (e.g. if we call sgdisk -e /dev/sda1,
it may fix partition table thus destroying the filesystem).
To prevent such cases, we do a dry-run at first and fail if
additional actions are scheduled.
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