libxl: idl: allow KeyedUnion members to be empty

libxl: idl: allow KeyedUnion members to be empty

This is useful when the key enum has an "invalid" option and avoids
the need to declare a dummy struct. Use this for domain_build_info
resulting in the generated API changing like so:
    --- tools/libxl/_libxl_BACKUP_types.h
    +++ tools/libxl/_libxl_types.h
    @@ -377,8 +377,6 @@ typedef struct libxl_domain_build_info {
                 const char * features;
                 libxl_defbool e820_host;
             } pv;
    -        struct {
    -        } invalid;
         } u;
     } libxl_domain_build_info;
     void libxl_domain_build_info_dispose(libxl_domain_build_info *p);

+ a related change to the JSON generation.

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