virtio: don't call device on !vm_running

On vm stop, virtio changes vm_running state
too soon, so callbacks can get envoked with
vm_running = false;

Cc: qemu-stable@nongnu.org
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>