util: check for an illegal character in a XML namespace prefix

When user tries to insert element metadata providing a namespace
declaration as well, currently we insert the element without any validation
check for XML prefix (if provided). The next VM start would then
fail with parse error. This patch fixes this issue by adding a call to
xmlValidateNCName function to check for illegal characters in the
prefix.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1143921

src/util/virxml.c

@@ -1066,6 +1066,12 @@ virXMLInjectNamespace(xmlNodePtr node,
 {
     xmlNsPtr ns;
 
+    if (xmlValidateNCName((const unsigned char *)key, 1) != 0) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("failed to validate prefix for a new XML namespace"));
+        return -1;
+    }
+
     if (!(ns = xmlNewNs(node, (const unsigned char *)uri, (const unsigned char *)key))) {
         virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                        _("failed to create a new XML namespace"));