Don't crash if there are no boot devices

If there are no disks found in the installer, disable the continue
button and properly show the message to the user instead of
crashing because we try to address a label which isn't created.

Change-Id: I9b7329a32b4dc6ff38d31b296deee22360512885
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1084528
Signed-off-by: Ryan Barry <rbarry@poseidon.phresus.priv>