vdpa_sim_net: Enable user to set mac address and mtu

Enable user to set the mac address and mtu so that each vdpa device
can have its own user specified mac address and mtu.

Now that user is enabled to set the mac address, remove the module
parameter for same.

And example of setting mac addr and mtu and view the configuration:
$ vdpa mgmtdev show
vdpasim_net:
  supported_classes net

$ vdpa dev add name bar mgmtdev vdpasim_net mac 00:11:22:33:44:55 mtu 9000

$ vdpa dev config show
bar: mac 00:11:22:33:44:55 link up link_announce false mtu 9000

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20211026175519.87795-6-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>