Preserve attributes when parsing AnyXml from XML

Preserve attributes when parsing AnyXml from XML

So far, anyxml nodes have been parsed in following pipeline:
 String -> DOM -> Stax -> String -> DOM
Now the parsing is simplified to:
 String -> DOM -> Stax -> DOM

Remove the unnecessary "back to string" step. This step is not just
unnecessary but also stripes down attributes and namespaces from AnyXml
node value.

JIRA: YANGTOOLS-923
Change-Id: If13ce9c7397c37b96a532090e67683a635ae14ef
Signed-off-by: Jakub Morvay <jmorvay@frinx.io>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>