mirror: fix throttling delay calculation

mirror: fix throttling delay calculation

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>
(cherry picked from commit cc8c9d6c6f28e4e376a6561a2a31524fd069bc2d)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>