[CORE] scheduler: Make APIs atomic for CPU on which called

This patch makes few vmm_scheduler APIs atomic for the CPU
on which they are called.

The rationale here is that vmm_scheduler has per-CPU context
for every host CPU hence, the per-CPU context on each CPU
should be accessed atomically using arch_cpu_irq_save() and
arch_cpu_irq_restore() APIs.

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