[CORE] vmm_loadbal: Create load balancing thread instead of orphan VCPU

This patch makes use of vmm_threads API in vmm_loadbal instead of
Orphan VCPU APIs for better readablity. This also shows our inclination
to use vmm_threads API over Orphan VCPU APIs.

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