credit2: Introduce a loadavg-based load balancer

credit2: Introduce a loadavg-based load balancer

This is a first-cut at getting load balancing.  I'm first working on
looking at behavior I want to get correct; then, once I know what kind
of behavior works well, then I'll work on getting it efficient.

The general idea is when balancing runqueues, look for the runqueue
whose loadavg is the most different from ours (higher or lower).
Then, look for a transaction which will bring the loads closest
together: either pushing a vcpu, pulling a vcpu, or swapping them.
Use the per-vcpu load to calculate the expected load after the
exchange.

The current algorithm looks at every combination, which is O(N^2).
That's not going to be suitable for workloads with large numbers of
vcpus (such as highly consolidated VDI deployments).  I'll make a more
efficient algorithm once I've experimented and determined what I think
is the best load-balancing behavior.

At the moment, balance from a runqueue every time the credit resets.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>