libxl: do not generate a destructor for data types which do not require one

libxl: do not generate a destructor for data types which do not require one

libxl_dominfo, libxl_poolinfo, libxl_vminfo, libxl_domain_build_state,
libxl_physinfo and libxl_sched_credit contain no members which require
destruction so omit the auto generated destructor.

As an exception continue to generate destructors for libxl_device_*
and libxl_{devtype}* even if they have no interesting members so that
all device types can be treated the same without special knowledge
about the type contents being required.

[PATCH 08 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>