LVM agents: Fix inability for services to relocate when devs fail

LVM was not able to properly shutdown when device failures caused
LVs to go missing. This meant that the service could not migrate
to another machine that (potentially) still had access to the
devices. Now we self-fence if we cannot shutdown the LVs (if
"self_fence" directive is set), allowing another machine to take
over the service.

Resolves: rhbz860981

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>