libxl: idl: always initialise the KeyedEnum keyvar in the member init function

libxl: idl: always initialise the KeyedEnum keyvar in the member init function

Previously we only initialised it if an explicit keyvar_init_val was
given but not if the default was implicitly 0.

In the generated code this only changes the unused libxl_event_init_type
function:

 void libxl_event_init_type(libxl_event *p, libxl_event_type type)
 {
+    assert(!p->type);
+    p->type = type;
     switch (p->type) {
     case LIBXL_EVENT_TYPE_DOMAIN_SHUTDOWN:
         break;

However I think it is wrong that this function is unused, this and
libxl_event_init should be used by libxl__event_new. As it happens
both are just memset to zero but for correctness we should use the
init functions (in case the IDL changes).

In the generator we also need to properly handle init_var == 0 which
the current if statements incorrectly treat as False. This doesn't
actually have any impact on the generated code.

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