[CORE] vmm_manager: Introduce common VCPU state change APIs

We simplify VCPU state changes by introducing one common VCPU
state change API (i.e. vmm_manager_vcpu_set_state()) and express
existing VCPU state change APIs in-terms of this common state
change API.

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