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: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <3f49c0ff601f27534d4536c87c00d01c233e067f.1513790495.git.alistair.francis@xilinx.com>
[CH: tweaked commit message]
Signed-off-by: Cornelia Huck <cohuck@redhat.com>