s390-virtio: use common features

We used to avoid enabling event_idx for virtio-blk devices via
s390-virtio, but we now have a workaround in place for guests trying
to use the device before setting DRIVER_OK. Therefore, let's add
DEFINE_VIRTIO_COMMON_FEATURES to the base device so all devices get
those common features - and make s390-virtio use the same mechanism
as the other transports do.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>