Revert "bus: do not unref the added child bus on realize"

Revert "bus: do not unref the added child bus on realize"

This is wrong.  object_finalize_child_property()'s unref balances the
ref in object_property_add_child().  qbus_realize's unref balances the
ref that was initially placed by object_new/object_initialize.

This reverts commit f3d58385a6d3d82f65db602c5506e2d3d8c82394.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180613172815.32738-4-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>