Interprete scoped key as nested tags

Current codes in xml_util.py does not support REST xml
so well when one tagname contains delimiter, it fails
to be interpreted. So this patch is to interprete one
tagname like "<a:b>1</a:b>" as a nested tag like
"<a><b>1</a></b>".

Fix bug:1205983

Change-Id: Ia3e1f4d6b6a739898b51bf052a784fa36ab7695e
Merge "Interprete scoped key as nested tags"