[libvirt] stream does not work in non-block mode with event loop implementation on Fibers

*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#