ethdev: expose device states

ethdev: expose device states

The hotplug API introduced multiple states for a device with possible
values defined internally, while the related field in struct rte_eth_dev
was made public.

Exposing those states improves consistency because applications have to
deal with the device list directly.

"DEV_DETACHED" is renamed "RTE_ETH_DEV_UNUSED" to better reflect that
the emptiness of a slot is not necessarily the result of detaching a
device.

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