[CORE] vmm_scheduler: Add new API vmm_scheduler_idle_vcpu()

This patch adds a new API vmm_scheduler_idle_vcpu() to determine
the idle VCPU for a give host CPU.

This new API will be very useful for load-balancing alogs who
require to sample the time spend by host CPU in idle state.

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