avocado.core.tree: Fix incorrect value/mux when using -m loc:path

When using ``avocado run -m $location:$path`` the last node of the
$location has to preserve value/mux of the root of the parsed $path
file. Previously it was just a fresh node and all values/tags were
missing. This patch only renames the original parsed root and extend
the tree of the remaining $location nodes.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/tree.py

@@ -456,7 +456,7 @@ def _create_from_yaml(path, cls_node=TreeNode):
 
     # Add prefix
     if using:
-        loaded_tree = cls_node(using.pop(), children=loaded_tree.children)
+        loaded_tree.name = using.pop()
         while True:
             if not using:
                 break