Host notifiers are guaranteed to be idle until the callbacks are
hooked up with virtio_queue_aio_set_host_notifier_handler(). They
thus don't need to be set or unset with the AioContext lock held.

Do this outside the critical section, like virtio-blk already
does : basically downgrading virtio_scsi_vring_init() to only
setup the host notifier and set the callback in the caller.

This will allow to batch addition/deletion of ioeventds in
a single memory transaction, which is expected to greatly
improve initialization time.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20210407143501.244343-4-groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>