virtio: use virtio_bus_set_host_notifier to start/stop ioeventfd

virtio: use virtio_bus_set_host_notifier to start/stop ioeventfd

ioeventfd_disabled was the only reason for the default
implementation of virtio_device_start_ioeventfd not to use
virtio_bus_set_host_notifier.  This is now fixed, and the sole entry
point to set up ioeventfd can be virtio_bus_set_host_notifier.

Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Cornelia Huck <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>