VT-d: fix interrupt remapping source validation for devices behind
legacy bridges

Using SVT_VERIFY_BUS here doesn't make sense; native Linux also
uses SVT_VERIFY_SID_SQ here instead.

This is XSA-33 / CVE-2012-5634.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/drivers/passthrough/vtd/intremap.c
+++ b/xen/drivers/passthrough/vtd/intremap.c
@@ -466,7 +466,7 @@ static void set_msi_source_id(struct pci_dev *pdev, struct iremap_entry *ire)
                 set_ire_sid(ire, SVT_VERIFY_BUS, SQ_ALL_16,
                             (bus << 8) | pdev->bus);
             else if ( pdev_type(seg, bus, devfn) == DEV_TYPE_LEGACY_PCI_BRIDGE )
-                set_ire_sid(ire, SVT_VERIFY_BUS, SQ_ALL_16,
+                set_ire_sid(ire, SVT_VERIFY_SID_SQ, SQ_ALL_16,
                             PCI_BDF2(bus, devfn));
         }
         break;