qemu_savevm_state will call all registered components with 3 phases: START,
PART, END. Only the PART phase is iterative.
In case of storage live migration we have lot more data to copy then memory
and usually the dirty rate is much less then memory dirty rate. I thought
about adding an iterative phase before PART for iterating massive data
transfer that we want to transfer before starting to transfer memory data.
The new phase will be iterative and will step to the next phase only when
all registered components will agree.