[CORE] More passive and lazy load balancer

This patch makes our existing crude load balancer more passive
and lazy running in background as Orphan VCPU on host CPU0.

Now the manager also provide APIs to change host CPU assigned
to a VCPU and uses this APIs in updated load balancer.

With this patch, we have UP Guest and SMP Guest booting on
SMP host (i.e. VExpress-A15) emulated by ARM fast models.

Also, having such a lazy load balancer running in background
will keep the load balancing overhead to minimum.

Signed-off-by: Anup Patel <anup@brainfault.org>