pc-bios/s390-ccw: Add virtio-net driver code

The driver provides the recv() and send() functions which will
be required by SLOF's libnet code for receiving and sending
packets.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1499863793-18627-11-git-send-email-thuth@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>