libxl: libxl_device_console.build_state is const

libxl: libxl_device_console.build_state is const

This field always refers to a libxl_domain_build_state structure which
is managed already by the caller, therefore it is incorrect to free it
as part of libxl_device_console_destroy.

(arguably this field should be a copy of, not a reference too, the original)

[PATCH 06 of 16 of
 libxl: autogenerate type definitions and destructor functions]

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