virDomainMemoryStats() gets those stats, if qemu is new enough to provide them, and if the guest cooperates to provide them. Well I use the latest QEMU 2.0.0 I have the latest CentOS 6.5 installed on the guest and unfortunately I can't get more stats that I have.
Which OS are you using in your guests?
Of course, you need to have the virtio balloon driver up and runnig in your guests. CentOS 6.5 x86_64 It has the balloon driver and it is up and running:
# modinfo virtio_balloon filename: /lib/modules/2.6.32-431.17.1.el6.x86_64/kernel/drivers/virtio/virtio_balloon.ko license: GPL description: Virtio balloon driver srcversion: BB9F75B2CAF3435CC507998 alias: virtio:d00000005v* depends: virtio,virtio_ring vermagic: 2.6.32-431.17.1.el6.x86_64 SMP mod_unload modversions