The second -device fails, because it refers to drive zwei, which got
silently ignored.
Make multiple drive definitions fail cleanly.
Unfortunately, there's code that relies on multiple drive definitions
being silently ignored: main() merrily adds default drives even when
the user already defined these drives. Fix that up.
Signed-off-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>