virtio/vhost-scsi: fix virtio-scsi/vhost-scsi child refcount in transports

virtio/vhost-scsi: fix virtio-scsi/vhost-scsi child refcount in transports

object_initialize() leaves the object with a refcount of 1.
object_property_add_child() adds its own reference which is dropped
again when the property is deleted.

The upshot of this is that we always have a refcount >= 1.  Upon hot
unplug the virtio-scsi/vhost-scsi child is not finalized!

Drop our reference after the child property has been added to the
parent.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 1312f12bcc8911ed99b67227fb9d1607295f71ed)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>