bus/vdev: implement plug operation

bus/vdev: implement plug operation

This method must be implemented to allow using a unified, generic API to
hotplug devices, including virtual ones.

VDEV devices actually exist unattached after performing a scan on the
rte_devargs list. As such it makes sense to be able to perform a device
hotplug afterward.

Finally, missing this generic interface forces the EAL to be dependent
on vdev-specific API, which hinders the plan of moving the vdev bus to
drivers/bus.

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