libxl: Do not trust frontend for channel in list

libxl: Do not trust frontend for channel in list

libxl_device_channel_list should not trust frontend-provided data.

So it needs to iterate using the /libxl paths, and read the backend
path out of /libxl.

However, it also filters out pure "consoles", which are channels
without a "name".  But the name was stored only in the frontend
directory, which the frontend can delete.

So store the name in the backend too.  (Ideally we would store it in
/libxl, where the backend can't write to it either, but
libxl__device_console_add not currently have access to the xenstore
transaction used by libxl__device_generic_add.  Protection against the
backend will come later, in XSA-178.)

Because the libxl paths are defined to be in terms of the frontend
device types, not the backend device types, it is no longer correct
for libxl__append_channel_list to take a type argument.  Abolish this
(with no functional effect).

This is part of XSA-175.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>