[CORE] Don't set new hcpu for a running VCPU

We still don't have clean way of migrating a running VCPU from
one hcpu to another. Trying to set new hcpu for running VCPU
seems to be most unstable for us.

This patch disable set new hcpu for running VCPU and also adds
comments for fixing this in future.

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