vtd: refuse to enable IOMMU if the PCI scan fails

This provides uniform behavior between Intel and AMD IOMMU initialization, and
is a requirement for PVHv2 Dom0, that depends on a working IOMMU plus the PCI
bus being scanned for devices.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>

xen/drivers/passthrough/vtd/iommu.c

@@ -2299,7 +2299,9 @@ int __init intel_vtd_setup(void)
     P(iommu_hap_pt_share, "Shared EPT tables");
 #undef P
 
-    scan_pci_devices();
+    ret = scan_pci_devices();
+    if ( ret )
+        goto error;
 
     ret = init_vtd_hw();
     if ( ret )