[CORE] Make vmm_manager_vcpu_set_hcpu() SMP safe

This patch makes vmm_manager_vcpu_set_hcpu() more atomic and SMP
safe so that when load balancer changes host CPU assigned to VCPU,
the VCPU transition from one host CPU to another occurs without
interruption and deadlocks.

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