netdevsim: Implement VFs

Allow creation of netdevsim ports for VFs along with allocations of
corresponding net devices and devlink ports.
Add enums and helpers to distinguish PFs' ports from VFs' ports.

Ports creation/deletion debugfs API intended to be used with physical
ports only.
VFs instantiation will be done in one of the next patches.

Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>