libsa/zfs: further improve handling of stale labels

[glebius]

Summary

Fix two problems with 6dd0803ffd31. First problem is that situation when
newer label was read before stale one, was handled differently to reverse
order case. Second problem is that vdev_free() would free the fully
initialized leaf vdev that carried stale label. In a case when vdev
carries a stale label, but is still referenced by a different label with
new a configuration, we don't want to free it, but rather insert it into
the new configuration.

Provide a helper function nvlist_find_vdev_guid() that checks presence
of certain GUID in a label.

In top level vdev store the GUID of vdev used to instantiate top vdev.

Cover all possible cases in the block in vdev_probe() where we encounter
a known configuration.  Make the diagnostic print more informative and
looking same regardless of probe order.  Make this whole block easier to
read reducing one level of indentation for a price of a single comparison
  at runtime.

[glebius]

Rewrite this revision based on feedback from Alexander.

Make sure that we don't free leaf vdevs with initialized v_phys_read so
that they can be reused if referenced by a new label. Make printing
better and summetrical against probing sequence.

[glebius]

Add missing STAILQ_REMOVE

[mav]

The logic of printing looks excessively complicated, but otherwise seems to make sense.

[imp]

I agree this is a bit ugly, but it seems a bit clearer than before...