qapi: Convert query-balloon

Please, note that some of the code supporting memory statistics is
still around (eg. virtio_balloon_receive_stats() and reset_stats()).

Also, the qmp_query_balloon() function is synchronous and thus doesn't
make any use of the (not fully working) monitor's asynchronous command
support (the old non-qapi implementation did).

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>