Assemble: avoid infinite loop when auto-assembling partial container.

Assemble: avoid infinite loop when auto-assembling partial container.

When auto-assembling we loop until we get no successes.

If a device is found that look like it is part of an already-existing
container, but we subsequently fail to add that device, then the fact
that the container is running looks like a success.  This can result
in infinite looping.
So if a container was already partially assemble, and is still only
partially assembled after we try to add devices, then don't treat that
as success.

Signed-off-by: NeilBrown  <neilb@suse.de>