ethdev: introduce device removal event

This new API allows reacting to a device removal.
A device removal is the sudden disappearance of a device from its
bus.

PMDs implementing support for this notification guarantee that the removal
of the underlying device does not incur a risk to the application.

In particular, Rx/Tx bursts and all other functions can still be called
(albeit likely returning errors) without triggering a crash, irrespective
of an application handling this event.

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>
Signed-off-by: Elad Persiko <eladpe@mellanox.com>