netdevsim: add SR-IOV functionality

dummy driver was extended with VF-related netdev APIs for testing
SR-IOV-related software.  netdevsim did not exist back then.
Implement SR-IOV functionality in netdevsim.  Notable difference
is that since netdevsim has no module parameters, we will actually
create a device with sriov_numvfs attribute for each netdev.
The zero MAC address is accepted as some HW use it to mean any
address is allowed.  Link state is also now validated.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>