*libvirt_stream_event_add_callback* creates *passthrough* variable
using *rb_ary_new2* and pass it to *virStreamEventAddCallback* at
*ext/libvirt/stream.c:272*.
After that ruby garbage collector removes this variable (I think
because there is no references for *passthrough* in ruby code).
https://bugzilla.redhat.com/show_bug.cgi?id=1787914#