netdevsim: move device registration and related code to bus.c

Move netdevsim device registration into bus.c and alongside with that
the related sysfs attributes. Introduce new struct nsim_bus_dev to
represent a netdevsim device on netdevsim bus.

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