vhost: Add worker backend callouts

vhost: Add worker backend callouts

This adds the vhost backend callouts for the worker ioctls added in the
6.4 linux kernel commit:

c1ecd8e95007 ("vhost: allow userspace to create workers")

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20231204231618.21962-2-michael.christie@oracle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>