This fixes several minor problems and adds some unit tests to ensure the correct behaviors
are preserved in the future.
We shouldn't be setting the target size from the size setter under any circumstances.
The size reflects reality for existing devices and the planned reality for non-existent
devices. Target size is only for resize of existing devices. The setter for one should not
modify the other.
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To view this pull request on github, visit
https://github.com/rhinstaller/blivet/pull/170