hw/s390x: Replace fprintf(stderr, "*\n" with qemu_log_mask()

One fprintf(stderr, was manually converted to a
qemu_log_mask(LOG_GUEST_ERROR,
Signed-off-by: NAlistair Francis <alistair.francis@xilinx.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-Id: <3f49c0ff601f27534d4536c87c00d01c233e067f.1513790495.git.alistair.francis@xilinx.com>
[CH: tweaked commit message]
Signed-off-by: NCornelia Huck <cohuck@redhat.com>

hw/s390x/virtio-ccw.c

@@ -426,8 +426,9 @@ static int virtio_ccw_cb(SubchDev *sch, CCW1 ccw)
                  * passes us zeroes for those we don't support.
                  */
                 if (features.features) {
-                    fprintf(stderr, "Guest bug: features[%i]=%x (expected 0)\n",
-                            features.index, features.features);
+                    qemu_log_mask(LOG_GUEST_ERROR,
+                                  "Guest bug: features[%i]=%x (expected 0)",
+                                  features.index, features.features);
                     /* XXX: do a unit check here? */
                 }
             }