Parallelize host evacuation even more

[last-genius]

With bab83d9, host evacuation was parallelized by grouping VMs
into batches, and starting a new batch once the previous one has finished.
This means that a single slow VM can potentially slow down the whole evacuation.

Add a new Tasks.wait_for_all_with_callback function that will
invoke a callback every time one of the tasks is
deemed non-pending. This will allow its users to:

1. track the progress of tasks within the submitted batch
2. schedule new tasks to replace the completed ones

Use the new Tasks.wait_for_all_with_callback in xapi_host to
schedule a new migration as soon as any of the previous ones have
finished, thus maintaining a constant flow of n migrations.

Additionally expose the evacuate-batch-size parameter in the CLI, this
was missed when it was originally added with the CLI setting it to 0
(pick the default) all the time.

===

Manually tested multiple times, confirmed to not break anything and to actually maintain a constant flow of migrations. This should greatly speed up host evacuations when there is a combination of bigger and smaller VMs (in terms of memory/disk, or VMs with some other reason for slow migration) on the host

[lindig]

This is an exciting direction. I implemented the batched migration and the reason it was batched was simplicity; a more pool-like behaviour clearly can improve performance.

[edwintorok]

We also have a TaskChains module in xapi_ha_vm_failover, although that one doesn't seem to limit the number of running tasks.
Eventually it'd be good if all these mechanisms were unified, but improving Host.evacuate is a good first step.