ethdev: add deferred intermediate device state

ethdev: add deferred intermediate device state

This device state means that the device is managed externally, by
whichever party has set this state (PMD or application).

Note: this new device state is only an information. The related device
structure and operators are still valid and can be used normally.

It is however made private by device management helpers within ethdev,
making the device invisible to applications.

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>