Update snapshot's format's exists flag based on its origin
Update snapshot's format's exists flag based on its origin

The snapshot's format exists once the snapshot is created iff the
origin's format exists. This applies to both thin and old
snapshots.

rhinstaller/blivet,jkonecny12/blivet,rhinstaller/blivet,vojtechtrefny/blivet,rvykydal/blivet,vojtechtrefny/blivet,jkonecny12/blivet,rvykydal/blivet