virtio/s390: add missing \n to end of dev_err message

Trival fix, dev_err message is missing a \n, so add it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Message-Id: <20160927200844.16008-1-colin.king@canonical.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>