netdevsim: add bus attributes to add new and delete devices

Add a way to add new netdevsim device on netdevsim bus and also to
delete existing netdevsim device from the bus. Track the bus devices
in using a list.

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