Force auto-convegence of live migration

Force auto-convegence of live migration

If a user chooses to turn on the auto-converge migration capability
these changes detect the lack of convergence and throttle down the
guest. i.e. force the VCPUs out of the guest for some duration
and let the migration thread catchup and help converge.

Verified the convergence using the following :
 - Java Warehouse workload running on a 20VCPU/256G guest(~80% busy)
 - OLTP like workload running on a 80VCPU/512G guest (~80% busy)

Sample results with Java warehouse workload : (migrate speed set to 20Gb and
migrate downtime set to 4seconds).

 (qemu) info migrate
 capabilities: xbzrle: off auto-converge: off  <----
 Migration status: active
 total time: 1487503 milliseconds
 expected downtime: 519 milliseconds
 transferred ram: 383749347 kbytes
 remaining ram: 2753372 kbytes
 total ram: 268444224 kbytes
 duplicate: 65461532 pages
 skipped: 64901568 pages
 normal: 95750218 pages
 normal bytes: 383000872 kbytes
 dirty pages rate: 67551 pages

 ---

 (qemu) info migrate
 capabilities: xbzrle: off auto-converge: on   <----
 Migration status: completed
 total time: 241161 milliseconds
 downtime: 6373 milliseconds
 transferred ram: 28235307 kbytes
 remaining ram: 0 kbytes
 total ram: 268444224 kbytes
 duplicate: 64946416 pages
 skipped: 64903523 pages
 normal: 7044971 pages
 normal bytes: 28179884 kbytes

Signed-off-by: Chegu Vinod <chegu_vinod@hp.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>