qemu: Do not ignore mandatory features in migration cookie

Due to "feature"/"features" nasty typo, any features marked as mandatory
by one side of a migration are silently considered optional by the other
side. The following is the code that formats mandatory features in
migration cookie:

    for (i = 0 ; i < QEMU_MIGRATION_COOKIE_FLAG_LAST ; i++) {
        if (mig->flagsMandatory & (1 << i))
            virBufferAsprintf(buf, "  <feature name='%s'/>\n",
                              qemuMigrationCookieFlagTypeToString(i));
    }

src/qemu/qemu_migration.c

@@ -817,7 +817,7 @@ qemuMigrationCookieXMLParse(qemuMigrationCookiePtr mig,
 
     /* Check to ensure all mandatory features from XML are also
      * present in 'flags' */
-    if ((n = virXPathNodeSet("./features", ctxt, &nodes)) < 0)
+    if ((n = virXPathNodeSet("./feature", ctxt, &nodes)) < 0)
         goto error;
 
     for (i = 0 ; i < n ; i++) {