Ignore open failures when relabelling disk images

NFS in root squash mode may prevent opening disk images to
determine backing store. Ignore errors in this scenario.

* src/security/security_selinux.c: Ignore open failures on disk
  images

src/security/security_selinux.c

@@ -482,7 +482,7 @@ SELinuxSetSecurityImageLabel(virSecurityDriverPtr drv,
 
     return virDomainDiskDefForeachPath(disk,
                                        allowDiskFormatProbing,
-                                       false,
+                                       true,
                                        SELinuxSetSecurityFileLabel,
                                        secdef);
 }