virsh: Adjust logic checks in virshUpdateDiskXML

Make it clearer that what we're trying to do is find @source and
@target_node so that the unattentive or code analysis utility
doesn't believe 'source' and 'target' could be found in the same
node element.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>

tools/virsh-domain.c

@@ -12476,10 +12476,9 @@ virshUpdateDiskXML(xmlNodePtr disk_node,
         if (tmp->type != XML_ELEMENT_NODE)
             continue;
 
-        if (virXMLNodeNameEqual(tmp, "source"))
+        if (!source && virXMLNodeNameEqual(tmp, "source"))
             source = tmp;
-
-        if (virXMLNodeNameEqual(tmp, "target"))
+        else if (!target_node && virXMLNodeNameEqual(tmp, "target"))
             target_node = tmp;
 
         /*