s390x: Switch to s390-ccw machine as default

We now finally have TCG support for the basic set of instructions necessary
to run the s390-ccw machine. That means in any aspect possible that machine
type is now superior to the legacy s390-virtio machine.

Switch over to the ccw machine as default. That way people don't get a halfway
broken machine with the s390x target.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>