hostdev: skip ACS check when using VFIO for device assignment

The ACS checks are meaningless when using the more modern VFIO driver
for device assignment since VFIO has its own more complete and exact
checks, but I didn't realize that when I added support for VFIO. This
patch eliminates the ACS check when preparing PCI devices for
assignment if VFIO is being used.

This resolves:

  https://bugzilla.redhat.com/show_bug.cgi?id=1256486

src/util/virhostdev.c

 /* virhostdev.c: hostdev management
  *
- * Copyright (C) 2006-2007, 2009-2013 Red Hat, Inc.
+ * Copyright (C) 2006-2007, 2009-2015 Red Hat, Inc.
  * Copyright (C) 2006 Daniel P. Berrange
  * Copyright (C) 2014 SUSE LINUX Products GmbH, Nuernberg, Germany.
  *
@@ -230,7 +230,6 @@ virHostdevGetPCIHostDeviceList(virDomainHostdevDefPtr *hostdevs, int nhostdevs)
             virObjectUnref(list);
             return NULL;
         }
-
         if (virPCIDeviceListAdd(list, dev) < 0) {
             virPCIDeviceFree(dev);
             virObjectUnref(list);
@@ -579,7 +578,7 @@ virHostdevPreparePCIDevices(virHostdevManagerPtr hostdev_mgr,
         struct virHostdevIsPCINodeDeviceUsedData data = {hostdev_mgr, dom_name,
                                                          usesVfio};
 
-        if (!virPCIDeviceIsAssignable(dev, strict_acs_check)) {
+        if (!usesVfio && !virPCIDeviceIsAssignable(dev, strict_acs_check)) {
             virReportError(VIR_ERR_OPERATION_INVALID,
                            _("PCI device %s is not assignable"),
                            virPCIDeviceGetName(dev));