[S390] Use s390_root_dev_* in kvm_virtio.

No need to define a static device for the kvm_s390 root device,
just use s390_root_dev_register().

This is needed for the bus_id rework

Acked-by: Carsten Otte <cotte@de.ibm.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>