migration/dirtyrate: Refactor dirty page rate calculation

abstract out dirty log change logic into function
global_dirty_log_change.

abstract out dirty page rate calculation logic via
dirty-ring into function vcpu_calculate_dirtyrate.

abstract out mathematical dirty page rate calculation
into do_calculate_dirtyrate, decouple it from DirtyStat.

rename set_sample_page_period to dirty_stat_wait, which
is well-understood and will be reused in dirtylimit.

handle cpu hotplug/unplug scenario during measurement of
dirty page rate.

export util functions outside migration.

Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <7b6f6f4748d5b3d017b31a0429e630229ae97538.1656177590.git.huangy81@chinatelecom.cn>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>