BUG-1275: Expose XmlStreamUtils.writeValue()

This exposes a utility method needed for RESTCONF datastore notification
delivery.

Change-Id: Ica14b6a11ecb95da306c37790e6a2fbf5533bfc1
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1275: Expose XmlStreamUtils.writeValue()"