[rhel6-branch][master] fix zipl boot loader upgrades

[David Cantrell <dcantrell redhat com>]

Strange bug, but legitimate.  I don't think this use case ever got much
testing because the code has been this way for a while.  The patch fixes
reading existing zipl configuration data so that boot loader changes
work during upgrade...and the reIPL device is set.

Tested on rhel6-branch, but suitable for master since the code is the
same for this stuff right now.  Sure am looking forward to a booty
overhaul.

-- 
David Cantrell <dcantrell redhat com>
Supervisor, Installer Engineering Team
Red Hat, Inc. | Honolulu, HI | UTC-10