Enabled qemu memory balloon stats

We can get vm memory stats from libvirt API
'virDomainMemoryStats', it help telemetry module
like as: Ceilometer to collect vm memory usage,
but by default the memory statistical feature is
disable in qemu, we need to add stats period
in order to enabled memory statistical.

blueprint enabled-qemu-memballoon-stats

Change-Id: Ia73b9b36e38a1d394d8d6bc98a03c3ca5f039949
Merge "Enabled qemu memory balloon stats"