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>
diff --git a/hmp.h b/hmp.h
index 09f3a01..d2c2d88 100644
--- a/hmp.h
+++ b/hmp.h
@@ -30,6 +30,7 @@
 void hmp_info_blockstats(Monitor *mon);
 void hmp_info_vnc(Monitor *mon);
 void hmp_info_spice(Monitor *mon);
+void hmp_info_balloon(Monitor *mon);
 void hmp_quit(Monitor *mon, const QDict *qdict);
 void hmp_stop(Monitor *mon, const QDict *qdict);
 void hmp_system_reset(Monitor *mon, const QDict *qdict);