The s390-virtio machine has been removed; remove the associated devices
as well.

hw/s390x/s390-virtio-bus.c and hw/s390x/s390-virtio-bus.h
have been deleted and removed from hw/s390x/Makefile.objs

virtio-size has no more meaning for the modern machine
and has been removed from helper.c and cpu.h

virtio-serial-s390 belonging to the old machine is
being removed from vl.c
Signed-off-by: NPierre Morel <pmorel@linux.vnet.ibm.com>
Acked-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>