KVM: x86: do not set st->preempted when going back to user space

Similar to the Xen path, only change the vCPU's reported state if the vCPU
was actually preempted.  The reason for KVM's behavior is that for example
optimistic spinning might not be a good idea if the guest is doing repeated
exits to userspace; however, it is confusing and unlikely to make a difference,
because well-tuned guests will hardly ever exit KVM_RUN in the first place.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>