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: Denis V. Lunev <den@openvz.org>
CC: Igor Redko <redkoi@virtuozzo.com>
CC: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>