migration: set dirty_pages_rate before autoconverge logic

Currently, a "period" in the RAM migration logic is at least a second
long and accounts for what happened since the last period (or the
beginning of the migration). The dirty_pages_rate counter is calculated
at the end this logic.

If the auto convergence capability is enabled from the start of the
migration, it won't be able to use this counter the first time around.
This calculates dirty_pages_rate as soon as a period is deemed over,
which allows for it to be used immediately.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>