virtio: balloon: Don't arm the stats timer of an inactive device

The update_stats_polling_interval() function arms the periodic timerfd
unconditionally. A PATCH /balloon/statistics after a device reset will
arm the timer. Given that there's nothing that consumes the event when
the timer fires, this can cause the main thread to spin unnecessarily.
Fix this by only arming the timer and notifying the guest when the
device is activated.

Signed-off-by: Ilias Stamatis <ilstam@amazon.com>