xen/pciback: Don't print scary messages when unsupported by hypervisor.

We print at the warninig level messages such as:
pciback 0000:90:00.5: MSI-X preparation failed (-38)

which is due to the hypervisor not supporting this sub-hypercall
(which was added in Xen 4.3).

Instead of having scary messages all the time - only have it
when the hypercall is actually supported.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/xen-pciback/pci_stub.c

@@ -118,7 +118,7 @@ static void pcistub_device_release(struct kref *kref)
 		int err = HYPERVISOR_physdev_op(PHYSDEVOP_release_msix,
 						&ppdev);
 
-		if (err)
+		if (err && err != -ENOSYS)
 			dev_warn(&dev->dev, "MSI-X release failed (%d)\n",
 				 err);
 	}
@@ -402,7 +402,7 @@ static int pcistub_init_device(struct pci_dev *dev)
 		};
 
 		err = HYPERVISOR_physdev_op(PHYSDEVOP_prepare_msix, &ppdev);
-		if (err)
+		if (err && err != -ENOSYS)
 			dev_err(&dev->dev, "MSI-X preparation failed (%d)\n",
 				err);
 	}