netdevsim: move netdev creation/destruction to dev probe

Remove the existing way to create netdevsim over rtnetlink and move the
netdev creation/destruction to dev probe, so for every probed port,
a netdevsim-netdev instance is created.

Adjust selftests to work with new interface.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>