vhost: get rid of vhost_poll_flush() wrapper

vhost_poll_flush() is a simple wrapper around vhost_work_dev_flush().
It gives wrong impression that we are doing some work over vhost_poll,
while in fact it flushes vhost_poll->dev.
It only complicate understanding of the code and leads to mistakes
like flushing the same vhost_dev several times in a row.

Just remove vhost_poll_flush() and call vhost_work_dev_flush() directly.

Signed-off-by: Andrey Ryabinin <arbn@yandex-team.com>
[merge vhost_poll_flush removal from Stefano Garzarella]
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20220517180850.198915-2-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>