disable guest-provided stats on "info balloon" command

The addition of memory stats reporting to the virtio balloon causes
the 'info balloon' command to become asynchronous.  This is a regression
because in some cases it can hang the user monitor.

This is an alternative to Adam Litke's patch. Adam's patch disabled the
corresponding (guest-visible) virtio feature bit, causing issues for migration.
Original discussion is available at:
http://marc.info/?l=qemu-devel&m=128448124328314&w=2Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Acked-by: Adam Litke <agl@us.ibm.com
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
(cherry picked from commit 07b0403d)

hw/virtio-balloon.c

@@ -29,6 +29,10 @@
 #include <sys/mman.h>
 #endif
 
+/* Disable guest-provided stats by now (https://bugzilla.redhat.com/show_bug.cgi?id=623903) */
+#define ENABLE_GUEST_STATS   0
+
+
 typedef struct VirtIOBalloon
 {
     VirtIODevice vdev;
@@ -83,12 +87,14 @@ static QObject *get_stats_qobject(VirtIOBalloon *dev)
                                   VIRTIO_BALLOON_PFN_SHIFT);
 
     stat_put(dict, "actual", actual);
+#if ENABLE_GUEST_STATS
     stat_put(dict, "mem_swapped_in", dev->stats[VIRTIO_BALLOON_S_SWAP_IN]);
     stat_put(dict, "mem_swapped_out", dev->stats[VIRTIO_BALLOON_S_SWAP_OUT]);
     stat_put(dict, "major_page_faults", dev->stats[VIRTIO_BALLOON_S_MAJFLT]);
     stat_put(dict, "minor_page_faults", dev->stats[VIRTIO_BALLOON_S_MINFLT]);
     stat_put(dict, "free_mem", dev->stats[VIRTIO_BALLOON_S_MEMFREE]);
     stat_put(dict, "total_mem", dev->stats[VIRTIO_BALLOON_S_MEMTOT]);
+#endif
 
     return QOBJECT(dict);
 }
@@ -214,7 +220,7 @@ static void virtio_balloon_to_target(void *opaque, ram_addr_t target,
         }
         dev->stats_callback = cb;
         dev->stats_opaque_callback_data = cb_data; 
-        if (dev->vdev.guest_features & (1 << VIRTIO_BALLOON_F_STATS_VQ)) {
+        if (ENABLE_GUEST_STATS && (dev->vdev.guest_features & (1 << VIRTIO_BALLOON_F_STATS_VQ))) {
             virtqueue_push(dev->svq, &dev->stats_vq_elem, dev->stats_vq_offset);
             virtio_notify(&dev->vdev, dev->svq);
         } else {