pc-bios/s390-ccw: update virtio implementation to allow up to 3 vrings

Add ability to work with up to 3 vrings, which is required for
virtio-scsi implementation.
Implement the optional cookie to speed up processing of virtio
notifications.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>