virtio-scsi-dataplane: fix memory leak in virtio_scsi_vring_init

if k->set_host_notifier failed, VirtIOSCSIVring *r will leak

Signed-off-by: Bo Su <subo7@huawei.com>
Message-Id: <1426671732-80213-1-git-send-email-subo7@huawei.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
