The throttling delay calculation was using an inaccurate sector count to
calculate the time to sleep. This broke rate-limiting for the block
mirror job.
Move the delay calculation into mirror_iteration() where we know how
many sectors were transferred. This lets us calculate an accurate delay
time.
Reported-by: Joaquim Barrera <jbarrera@ac.upc.edu> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>