Make our PV drivers work  with older hosts that do not recognize the new PV driver protocol. 

Signed-off-by: K. Y. Srinivasan <ksrinivasan@xxxxxxxxxx>

Index: xen-4.0.0-testing/unmodified_drivers/linux-2.6/platform-pci/platform-pci.c
===================================================================
--- xen-4.0.0-testing.orig/unmodified_drivers/linux-2.6/platform-pci/platform-pci.c	2009-12-22 06:23:01.000000000 -0700
+++ xen-4.0.0-testing/unmodified_drivers/linux-2.6/platform-pci/platform-pci.c	2010-01-06 12:50:53.000000000 -0700
@@ -284,7 +284,10 @@
 
 	if (magic != XEN_IOPORT_MAGIC_VAL) {
 		dev_err(dev, "invalid magic %#x", magic);
-		return -ENODEV;
+		/* 
+		 * Older backend; just return 0 to be compatible.
+		 */
+		return 0;
 	}
 
 	protocol = inb(XEN_IOPORT_PROTOVER);