virtio-serial: fix virtio-serial child refcount in transports

virtio-serial: fix virtio-serial 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-serial child is not finalized!

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

Signed-off-by: Gonglei <[email protected]>
Reviewed-by: Cornelia Huck <[email protected]>
Cc: [email protected]
Signed-off-by: Paolo Bonzini <[email protected]>