xml FEATURE support for special xml attr prefix

Which is interpreted as part of the name, not
as a prefix.
Refs cesnet/libnetconf2#318
diff --git a/src/tree_data.c b/src/tree_data.c
index a2c3ef3..29d2112 100644
--- a/src/tree_data.c
+++ b/src/tree_data.c
@@ -1417,6 +1417,14 @@
         return LY_EVALID;
     }
 
+    if ((pref_len == 3) && !strncmp(prefix, "xml", 3)) {
+        /* not a prefix but special name */
+        name = prefix;
+        name_len += 1 + pref_len;
+        prefix = NULL;
+        pref_len = 0;
+    }
+
     /* get the module */
     if (module_name) {
         mod_len = strlen(module_name);
@@ -1459,6 +1467,14 @@
         return LY_EVALID;
     }
 
+    if ((pref_len == 3) && !strncmp(prefix, "xml", 3)) {
+        /* not a prefix but special name */
+        name = prefix;
+        name_len += 1 + pref_len;
+        prefix = NULL;
+        pref_len = 0;
+    }
+
     /* set value if none */
     if (!value) {
         value = "";