Previously we made all the default encoding non-permanent by XML objects
before generating the XML, making changes to the copies, and restoring
to the old state after the XML is returned to the user.
This allows us to call start_install multiple times with the same Guest
object and not alter it's original config... but that feature isn't really
useful anymore, and this behavior makes the 'customize before install'
dialog difficult to handle.
So drop most of it, and fix some of the minor fallout.
86f1133777