qapi: fix memleak by adding implict struct functions in dealloc visitor

Otherwise member "base" is leaked in a qapi_free_STRUCTURE() call.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1383676551-18806-2-git-send-email-xiawenc@linux.vnet.ibm.com
Cc: qemu-stable@nongnu.org
Signed-off-by: Anthony Liguori <aliguori@amazon.com>
