virtio-scsi-dataplane: fix memory leak for VirtIOSCSIVring

VirtIOSCSIVring which allocated in virtio_scsi_vring_init
should be free when dataplane has been stopped or failed to start.

Signed-off-by: Ting Wang <kathy.wangting@huawei.com>
Message-Id: <1427355752-25844-1-git-send-email-kathy.wangting@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>