qemu: snapshot: Don't update current snapshot until we're done

qemu: snapshot: Don't update current snapshot until we're done

Since commit f105627992e 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: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>