libxl: generate destructors for each libxl defined type

libxl: generate destructors for each libxl defined type

I chose the name "_destroy" rather than "_free" because the destructor
functions will free only the members of a type recursively but will
not free the actual type structure itself. The allocation of the type
is typically done by the caller and may not be a single allocation,
e.g. lists/arrays of types or embedded in other structures etc.

The exceptions to this rule are libxl_string_list_destroy and
libxl_key_value_list_destroy but I'm not 100% convinced they are
exceptions (since they are kind-of opaque) and I couldn't see a
cleanerway to express this concept. I have made a best effort attempt
to implement these functions sanely but since as far as I can tell
nothing in the current code base ever sets
libxl_domain_create_info.{xsdata,platformdata} I'm flying somewhat
blind.

[PATCH 05 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>