922806: Fix RHEL 5 firstboot issue with backButton.

Disabling the next/back buttons implemented in commit 1f70a077 appears
to be RHEL 6 specific.

On RHEL 5, the interface provided is a GtkNotebook which has no
next/back buttons on it. However we do have a reference to a parent
which does, and we can use instead.
Merge pull request #626 from candlepin/dgoodwin/el5-fb-backbutton

922806: Fix RHEL 5 firstboot issue with backButton.