vhost-user-snd: correct the calculation of config_size

vhost-user-snd: correct the calculation of config_size

Use virtio_get_config_size() rather than sizeof(struct
virtio_snd_config) for the config_size in the vhost-user-snd frontend.
The frontend shall rely on device features for the size of the device
configuration space. The presence of `controls` in the config space
depends on VIRTIO_SND_F_CTLS according to the specification (v1.3):
`
5.14.4 Device Configuration Layout
...

controls
(driver-read-only) indicates a total number of all available control
elements if VIRTIO_SND_F_CTLS has been negotiated.
`
This fixes an issue introduced by commit ab0c7fb2 ("linux-headers:
update to current kvm/next") in which the optional field `controls` is
added to the virtio_snd_config structure. This breaks vhost-user-device
backends that do not implement the `controls` field.

Fixes: ab0c7fb22b ("linux-headers: update to current kvm/next")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2805
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@redhat.com>
Message-Id: <20250217131255.829892-1-mvaralar@redhat.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Dorinda Bassey <dbassey@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>