netconf: drop rollback to last persisted if mem rollback fails

When memory rollback fails it is likely to be due to a malfunction
or a state that we probably can't handle. The previous behavior was
to let that on the hands of the oVirt fencing. This patch restores
the known and expected behavior.

Change-Id: Iefcad39b37066c0e52a1da85144a074c63bed174
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21867
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>