pc-bios/s390-ccw: Add code for virtio feature negotiation

The upcoming virtio-net driver needs to negotiate some features,
so we need the possibility to do this in the core virtio code.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1499863793-18627-8-git-send-email-thuth@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>