libxl: remove libxl_device_console_add.

libxl: remove libxl_device_console_add.

It has no callers, the only code which adds consoles in internal to libxl and
uses libxl__device_console_add directly.

Rather than worrying about what the public API should look like in this case
simply remove it, adding new APIs is much easier than fixing broken ones...

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>