conf: use virXMLPropString for IOMMU def parsing

XPath is good for random search of elements, not for accessing
attributes of one node.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/conf/domain_conf.c

@@ -14429,6 +14429,7 @@ virDomainIOMMUDefParseXML(xmlNodePtr node,
 {
     virDomainIOMMUDefPtr iommu = NULL, ret = NULL;
     xmlNodePtr save = ctxt->node;
+    xmlNodePtr driver;
     char *tmp = NULL;
     int val;
 
@@ -14450,8 +14451,9 @@ virDomainIOMMUDefParseXML(xmlNodePtr node,
 
     iommu->model = val;
 
+    if ((driver = virXPathNode("./driver", ctxt))) {
         VIR_FREE(tmp);
-    if ((tmp = virXPathString("string(./driver/@intremap)", ctxt))) {
+        if ((tmp = virXMLPropString(driver, "intremap"))) {
             if ((val = virTristateSwitchTypeFromString(tmp)) < 0) {
                 virReportError(VIR_ERR_XML_ERROR, _("unknown intremap value: %s"), tmp);
                 goto cleanup;
@@ -14460,7 +14462,7 @@ virDomainIOMMUDefParseXML(xmlNodePtr node,
         }
 
         VIR_FREE(tmp);
-    if ((tmp = virXPathString("string(./driver/@caching_mode)", ctxt))) {
+        if ((tmp = virXMLPropString(driver, "caching_mode"))) {
             if ((val = virTristateSwitchTypeFromString(tmp)) < 0) {
                 virReportError(VIR_ERR_XML_ERROR, _("unknown caching_mode value: %s"), tmp);
                 goto cleanup;
@@ -14468,7 +14470,7 @@ virDomainIOMMUDefParseXML(xmlNodePtr node,
             iommu->caching_mode = val;
         }
         VIR_FREE(tmp);
-    if ((tmp = virXPathString("string(./driver/@iotlb)", ctxt))) {
+        if ((tmp = virXMLPropString(driver, "iotlb"))) {
             if ((val = virTristateSwitchTypeFromString(tmp)) < 0) {
                 virReportError(VIR_ERR_XML_ERROR, _("unknown iotlb value: %s"), tmp);
                 goto cleanup;
@@ -14477,13 +14479,14 @@ virDomainIOMMUDefParseXML(xmlNodePtr node,
         }
 
         VIR_FREE(tmp);
-    if ((tmp = virXPathString("string(./driver/@eim)", ctxt))) {
+        if ((tmp = virXMLPropString(driver, "eim"))) {
             if ((val = virTristateSwitchTypeFromString(tmp)) < 0) {
                 virReportError(VIR_ERR_XML_ERROR, _("unknown eim value: %s"), tmp);
                 goto cleanup;
             }
             iommu->eim = val;
         }
+    }
 
     ret = iommu;
     iommu = NULL;