[CORE] Avoid using vmm_manager_vcpu_iterate() in load balancing

The vmm_manager_vcpu_iterate() acquires manager lock and then
iterates over each VCPU. This API is good for stuff like statistic
gathering or listing or count specific types of VCPU but not good
for heavy task like moving VCPU from one host CPU to another.

In future, we would only use vmm_manager_vcpu_iterate() for very
lightweight task as described above.

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