[CORE] move VCPU state to atomic_t.

This improve a bit the overal speed by making VCPU 'state'
as atomic_t.

The rationale here is to improve speed of read access to VCPU
'state' by make it atomic_t so that vmm_manager_vcpu_get_state()
becomes faster. We will still need to use 'sched_lock' for updating
VCPU 'state' because state-level statistics get updated whenver
state changes.

In other words, we still use 'sched_lock' for accessing members
of scheduling context in VCPU context except that for reading VCPU
'state' we will do simple atomic read opertion.

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>