Note that we mark as preempted only when vcpu's task state was
Running during preemption.

Thanks Jiannan, Avi for preemption notifier ideas. Thanks Gleb, PeterZ
for their precious suggestions. Thanks Srikar for an idea on avoiding
rcu lock while checking task state that improved overcommit numbers.
Reviewed-by: NChegu Vinod <chegu_vinod@hp.com>
Reviewed-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NRaghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Signed-off-by: NGleb Natapov <gleb@redhat.com>