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/qmp-commands.hx b/qmp-commands.hx
index e99c240..8ef6ca2 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -1989,3 +1989,8 @@
 
 EQMP
 
+    {
+        .name       = "query-balloon",
+        .args_type  = "",
+        .mhandler.cmd_new = qmp_marshal_input_query_balloon,
+    },