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.
Edited by Marc-André Lureau