nixpart0: Fix race condition in formatting devices

This adds a backport of rhinstaller/blivet#39 to the pinned blivet
version 0.17, it's addressing the following upstream bug:

https://bugzilla.redhat.com/show_bug.cgi?id=1196397



It has been reported at aszlig/nixpart#7 and tested by @manveru (the
issue reporter), thanks a lot.

Thanks also to @domenkozar for finding the upstream issue.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Reported-by: Michael Fellinger <m.fellinger@gmail.com>
Fixes: aszlig/nixpart#7