Correct failure to attach the PV block front device on Citrix

XenServer configurations that advertise the multi-page ring extension,
but only allow a single page of ring space.

sys/dev/xen/blkfront/blkfront.c:
	If only one page of ring space is being used, do not publish
	in the XenStore the number of pages in use (1), via either
	of the supported multi-page ring extension schemes.

	Single page operation is the same with or without the
	ring-page extension being negotiated.   Relying on the
	legacy behavior avoids an incompatible difference in how
	the two ring-page extension schemes that are out in the
	wild, deal with the base case of a single page.  The
	Amazon/Red Hat drivers use the same XenStore variable as
	if the extension was not negotiated.  The Citrix drivers
	assume the new ring reference XenStore variables will be
	available

Reported by:	Oliver Schonefeld <[email protected]>
MFC after:	3 days

sys/dev/xen/blkfront/blkfront.c

@@ -698,21 +698,25 @@ blkfront_initialize(struct xb_softc *sc)
 		return;
 
 	/* Support both backend schemes for relaying ring page limits. */
-	error = xs_printf(XST_NIL, node_path,
-			 "num-ring-pages","%u", sc->ring_pages);
-	if (error) {
-		xenbus_dev_fatal(sc->xb_dev, error,
-				 "writing %s/num-ring-pages",
-				 node_path);
-		return;
-	}
-	error = xs_printf(XST_NIL, node_path,
-			 "ring-page-order","%u", fls(sc->ring_pages) - 1);
-	if (error) {
-		xenbus_dev_fatal(sc->xb_dev, error,
-				 "writing %s/ring-page-order",
-				 node_path);
-		return;
+	if (sc->ring_pages > 1) {
+		error = xs_printf(XST_NIL, node_path,
+				 "num-ring-pages","%u", sc->ring_pages);
+		if (error) {
+			xenbus_dev_fatal(sc->xb_dev, error,
+					 "writing %s/num-ring-pages",
+					 node_path);
+			return;
+		}
+
+		error = xs_printf(XST_NIL, node_path,
+				 "ring-page-order", "%u",
+				 fls(sc->ring_pages) - 1);
+		if (error) {
+			xenbus_dev_fatal(sc->xb_dev, error,
+					 "writing %s/ring-page-order",
+					 node_path);
+			return;
+		}
 	}
 
 	error = xs_printf(XST_NIL, node_path,