bus/vdev: fix hotplug twice

bus/vdev: fix hotplug twice

In case vdev was already probed, it shouldn't be probed again,
and it should return -EEXIST as error.
There are some checks in vdev_probe() and insert_vdev(),
but a check was missing in vdev_plug().
The check is moved in vdev_probe_all_drivers() which is called
in all code paths.

Fixes: e9d159c3d534 ("eal: allow probing a device again")
Cc: stable@dpdk.org
Signed-off-by: Raslan Darawsheh <rasland@mellanox.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>