GuestMemory: Start memory stats collection on initialization

Specify update rate of memory stats of virtio balloon driver using
new libvirt API virDomainSetMemoryStatsPeriod.

Change-Id: If90ff472d57d54edc6f22baf2a41322cb9e79448
Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-on: http://gerrit.ovirt.org/19199
Reviewed-by: Adam Litke <agl@us.ibm.com>
Tested-by: Adam Litke <agl@us.ibm.com>