virtio-snd: handle 5.14.6.2 for PCM_INFO properly

The section 5.14.6.2 of the VIRTIO spec says:

  5.14.6.2 Driver Requirements: Item Information Request

  - The driver MUST NOT set start_id and count such that start_id +
    count is greater than the total number of particular items that is
    indicated in the device configuration space.

  - The driver MUST provide a buffer of sizeof(struct virtio_snd_hdr) +
    count * size bytes for the response.

While we performed some check for the second requirement, it failed to
check for integer overflow.

Add also a check for the first requirement, which should limit exposure
to any overflow, since realistically the number of streams will be low
enough in value such that overflow is improbable.

Cc: qemu-stable@nongnu.org
Reported-by: 罗铭源 <myluo24@m.fudan.edu.cn>
Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260220-virtio-snd-series-v1-3-207c4f7200a2@linaro.org>