net: dummy: Introduce dummy virtual functions

The idea for this was born when testing VF support in iproute2 which was
impeded by hardware requirements. In fact, not every VF-capable hardware
driver implements all netdev ops, so testing the interface is still hard
to do even with a well-sorted hardware shelf.

To overcome this and allow for testing the user-kernel interface, this
patch allows to turn dummy into a PF with a configurable amount of VFs.

Since my patch series 'bus-agnostic-num-vf' has been accepted,
implementing the required interfaces is pretty straightforward: Iff
'num_vfs' module parameter was given a value >0, a dummy bus type is
being registered which implements the 'num_vf()' callback. Additionally,
a dummy parent device common to all dummy devices is registered which
sits on the above dummy bus.

Joint work with Sabrina Dubroca.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>