Add channel # to rhs_write XML messages

[analog-cbarber]

Currently messages coming from the trace, log and write RHS functions all look the same to print handlers.

As such the "channel" functions more like a on/off flag rather than what you would normally call a channel.

If you add a channel attribute to rhs_write XML messages when they come from log, then print handlers will be able to use that to determine how to display or handle the message.

You might want to do something similar with trace