qapi: generate list struct and visit_list for enum

qapi: generate list struct and visit_list for enum

Currently, if we define an 'enum' and use it in one command's
data, list struct for enum could not be generated, but it's
used in qmp function.

For example: KeyCodesList could not be generated.
>>> qapi-schema.json:
{ 'enum': 'KeyCodes',
  'data': [ 'shift', 'alt' ... ] }
{ 'command': 'sendkey',
  'data': { 'keys': ['KeyCodes'], '*hold-time': 'int' } }

>>> qmp-command.h:
void qmp_sendkey(KeyCodesList * keys, bool has_hold_time, int64_t
hold_time, Error **errp);

This patch lets qapi generate list struct and visit_list for enum.

Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>