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 <stefanha@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>