virtio: Always reset vhost devices

virtio: Always reset vhost devices

Requiring `vhost_started` to be true for resetting vhost devices in
`virtio_reset()` seems like the wrong condition: Most importantly, the
preceding `virtio_set_status(vdev, 0)` call will (for vhost devices) end
up in `vhost_dev_stop()` (through vhost devices' `.set_status`
implementations), setting `vdev->vhost_started = false`.  Therefore, the
gated `vhost_reset_device()` call is unreachable.

`vhost_started` is not documented, so it is hard to say what exactly it
is supposed to mean, but judging from the fact that `vhost_dev_start()`
sets it and `vhost_dev_stop()` clears it, it seems like it indicates
whether there is a vhost back-end, and whether that back-end is
currently running and processing virtio requests.

Making a reset conditional on whether the vhost back-end is processing
virtio requests seems wrong; in fact, it is probably better to reset it
only when it is not currently processing requests, which is exactly the
current order of operations in `virtio_reset()`: First, the back-end is
stopped through `virtio_set_status(vdev, 0)`, then we want to send a
reset.

Therefore, we should drop the `vhost_started` condition, but in its
stead we then have to verify that we can indeed send a reset to this
vhost device, by not just checking `k->get_vhost != NULL` (introduced by
commit 95e1019a4a9), but also that the vhost back-end is connected
(`hdev = k->get_vhost(); hdev != NULL && hdev->vhost_ops != NULL`).

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20240723163941.48775-3-hreitz@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>