Fix "disk zap" sgdisk invocation

If the metadata on the disk is truly invalid, sgdisk would fail to zero
it in one go, because --mbrtogpt apparently tried to operate on the
metadata it read before executing --zap-all.

Splitting this up into two separate invocations to first zap everything
and then clear it properly fixes this issue.
Merge pull request #200 from l-mb/master

Fix "disk zap" sgdisk invocation