kvm,powerpc: Serialize wq active checks in ops->vcpu_kick

kvm,powerpc: Serialize wq active checks in ops->vcpu_kick

Particularly because kvmppc_fast_vcpu_kick_hv() is a callback,
ensure that we properly serialize wq active checks in order to
avoid potentially missing a wakeup due to racing with the waiter
side.

Signed-off-by: Davidlohr Bueso <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>