[CORE] vmm_vcpu_irq: Fix vmm_vcpu_irq_process() and consmetic changes

The vmm_vcpu_irq_process() was using index 'i' in the statement
"arch_atomic_cmpxchg(&vcpu->irqs.irq[i].assert, ....)" which is wrong
because we want to change assert state of 'irq_no'. This patch fixes
this issues in vmm_vcpu_irq_process().

In addition, this patch does few cosmetic changes related to usage of
arch_atomic_cmpxchg() function.

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