qapi-event: Simplify visit of non-implicit data

Commit 7ce106a9 documented why we don't generated a visit_type_FOO()
for implicit types; and therefore events with an anonymous type for
'data' have to open-code a visit.  Note that the open-coded visit in
qapi-event.c is slightly different from what is done in
qapi-visit.c for normal types, in part because we don't have to
check for *obj being NULL or free things on error.  But where the
type is not implicit, it is nicer to reuse the normal visit instead
of open-coding a duplicate.

At the moment, the only event with a non-implicit 'data' is in the
testsuite, where test-qapi-event.c changes as follows:

|@@ -155,6 +155,7 @@ void qapi_event_send___org_qemu_x_event(
|     __org_qemu_x_Struct param = {
|         __org_qemu_x_member1, (char *)__org_qemu_x_member2, has_q_wchar_t, q_wchar_t
|     };
|+    __org_qemu_x_Struct *arg = &param;
|
|     emit = qmp_event_get_func_emit();
|     if (!emit) {
|@@ -164,16 +165,7 @@ void qapi_event_send___org_qemu_x_event(
|     qmp = qmp_event_build_dict("__ORG.QEMU_X-EVENT");
|
|     v = qmp_output_visitor_new(&obj);
|-
|-    visit_start_struct(v, "__ORG.QEMU_X-EVENT", NULL, 0, &err);
|-    if (err) {
|-        goto out;
|-    }
|-    visit_type___org_qemu_x_Struct_members(v, &param, &err);
|-    if (!err) {
|-    if (!err) {
|-        visit_check_struct(v, &err);
|-    }
|-    visit_end_struct(v, NULL);
|+    visit_type___org_qemu_x_Struct(v, "__ORG.QEMU_X-EVENT", &arg, &err);
|     if (err) {
|         goto out;
|     }

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1468468228-27827-8-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
