vdpa_sim: Skip typecasting from void*

Typecasting from void* to virtio_net_config* is not needed.
Remove it.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Link: https://lore.kernel.org/r/20210213183951.248324-1-parav@nvidia.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>