zb: wrap serde_xml_rs::Error in zbus::Error

zbus has a single Error type that all APIs use, except a few functions in the xml module. Wrap the underlying serde_xml_rs error, so its callers don't have to deal with serde_xml directly, and zbus-xml can more freely return different error types in the future.

This is an API break, but with moderate impact, since callers usually can't do much with the XML error.