virtio-balloon: add 'available' counter

The patch for the kernel part is in linux-next already:
commit ac88e7c908b920866e529862f2b2f0129b254ab2
    Author: Igor Redko <redkoi@virtuozzo.com>
    Date:   Thu Feb 18 09:23:01 2016 +1100

    virtio_balloon: export 'available' memory to balloon statistics

    Add a new field, VIRTIO_BALLOON_S_AVAIL, to virtio_balloon memory
    statistics protocol, corresponding to 'Available' in /proc/meminfo.
Signed-off-by: NDenis V. Lunev <den@openvz.org>
CC: Igor Redko <redkoi@virtuozzo.com>
CC: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-balloon.c

@@ -53,6 +53,7 @@ static const char *balloon_stat_names[] = {
    [VIRTIO_BALLOON_S_MINFLT] = "stat-minor-faults",
    [VIRTIO_BALLOON_S_MEMFREE] = "stat-free-memory",
    [VIRTIO_BALLOON_S_MEMTOT] = "stat-total-memory",
+   [VIRTIO_BALLOON_S_AVAIL] = "stat-available-memory",
    [VIRTIO_BALLOON_S_NR] = NULL
 };
 

include/standard-headers/linux/virtio_balloon.h

@@ -51,7 +51,8 @@ struct virtio_balloon_config {
 #define VIRTIO_BALLOON_S_MINFLT   3   /* Number of minor faults */
 #define VIRTIO_BALLOON_S_MEMFREE  4   /* Total amount of free memory */
 #define VIRTIO_BALLOON_S_MEMTOT   5   /* Total amount of memory */
-#define VIRTIO_BALLOON_S_NR       6
+#define VIRTIO_BALLOON_S_AVAIL    6   /* Amount of available memory in guest */
+#define VIRTIO_BALLOON_S_NR       7
 
 /*
  * Memory statistics structure.