Since commit f1056279 we store whether a snapshot is current globally
rather than locally in the snapshot object.

This means that we don't have to unset the current snapshot prior to
taking/reverting the snapshot and we can do it only when everything is
done successfully.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>