netdevsim: move vfconfig to nsim_dev

When netdevsim got split into the faux bus vfconfig ended
up in the bus device (think pci_dev) which is strange because
it contains very networky not to say netdevy information.
Move it to nsim_dev, which is the driver "priv" structure
for the device.

To make sure we don't race with probe/remove take
the device lock (much like PCI).

While at it remove the NULL-checking of vfconfigs.
It appears to be pointless.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>