Refactor: various: Replace XML_ATTR_UUID with XML_ATTR_ID internally

XML_ATTR_UUID has always been defined as "id", the same value as
XML_ATTR_ID. Keeping both and using them roughly interchangeably creates
confusion, especially since for nodes the XML_ATTR_UUID is not a true
UUID.

There are still many comments and other variable names that refer to
"uuid" or "UUID" in places where we've used XML_ATTR_UUID. That's going
to be painful to sort out and may be better left for a major version
bump.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>