libxl: make libxl_device_console internal

libxl: make libxl_device_console internal

consoles are not directly exposed to users of the library and there are no API
functions for manipluating them (only the console_exec function). Rather than
commit to a particular API now make the type internal.

When a user does come along it is much easier to add a completely new API
rather than to fix an existing broken one. It's easier to do this in a manner
which users of the library can cope with in a compatible way e.g. adding a new
API is easier to check for with ./configure.

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