devargs: introduce removal function

devargs: introduce removal function

Hotplug support introduces the possibility of removing devices from the
system. Allocated resources must be freed.

Extend the rte_devargs API to allow freeing allocated resources.

This API is experimental and bound to change. It is currently designed
as a symetrical to rte_eal_devargs_add(), but the latter will evolve
shortly anyway.

Its DEVTYPE parameter is currently only used to specify scan policies,
and those will evolve in the next release. This evolution should
rationalize the rte_devargs API.

As such, the proposed API here is not the most convenient, but is
taylored to follow the current design and integrate easily with its main
use within rte_eal_hotplug_* functions.

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