[CORE] Do VCPU state transitions in vmm_scheduler only

Till now, vmm_manager_vcpu_state_change() used to control
VCPU state transitions and notify vmm_scheduler using
vmm_scheduler_state_change() API.

This unifies the VCPU state transition and moves it to
vmm_scheduler_state_change() API.

The vmm_manager_vcpu_state_change() would be still required
for SMP to do vmm_scheduler_state_change() on appropriate
host CPU.

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