tests: Enable the drive_del test also on s390x

We can use the drive_del test on s390x, too, to check that adding and
deleting also works fine with the virtio-ccw bus. But we have to make
sure that we use the devices with the "-ccw" suffix instead of the
"-pci" suffix for the virtio-ccw transport on s390x. Introduce a helper
function called qvirtio_get_dev_type() that returns the correct string
for the current architecture.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1504190408-11143-1-git-send-email-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>