vhost: fix poll coding style

We use 3 coding styles in this struct. Switch to just tabs.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210525174733.6212-5-michael.christie@oracle.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>