balloon: don't set balloon && polling interval at start when livemigrate

We don't need to set balloon value and polling interval when a vm is coming from a livemigrate.
(Values are keep in guest memory)

So with autoballooning, this avoid to set the ballon size at ballon_min value when the vm is migrated

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>