H2: Log H2 upgrade to the session vsl instead of the 0 vxid

For PU upgrade, at the time of this log, the req->vsl is not assigned a vxid
yet, and thus the line would be logged for vxid 0, which doen't make much
sense. In all cases, it is better to log it on the sess vsl instead.

Noticed while reading a vtc output.

bin/varnishd/http2/cache_http2_session.c

@@ -350,15 +350,15 @@ h2_ou_session(struct worker *wrk, struct h2_sess *h2,
 void
 H2_PU_Sess(struct worker *wrk, struct sess *sp, struct req *req)
 {
-	VSLb(req->vsl, SLT_Debug, "H2 Prior Knowledge Upgrade");
+	VSL(SLT_Debug, sp->vxid, "H2 Prior Knowledge Upgrade");
 	req->err_code = H2_PU_MARKER;
 	SES_SetTransport(wrk, sp, req, &HTTP2_transport);
 }
 
 void
 H2_OU_Sess(struct worker *wrk, struct sess *sp, struct req *req)
 {
-	VSLb(req->vsl, SLT_Debug, "H2 Optimistic Upgrade");
+	VSL(SLT_Debug, sp->vxid, "H2 Optimistic Upgrade");
 	req->err_code = H2_OU_MARKER;
 	SES_SetTransport(wrk, sp, req, &HTTP2_transport);
 }