virtio-snd: use 'data buffer' instead of 'buffer'

[MatiasVara]

This commit replaces the wording 'buffer' with 'data buffer' when referring to the payload of PCM messages. The virtio specification defines the word buffer as a set of zero or more device-readable physically-contiguous elements followed by zero or more physically-contiguous device-writable elements. In the virtio-sound section, the word is used only for the data or payload in a request. In this case, the data buffer is sent together with a header, i.e., device-readable, and the status, i.e., device-writable. To not get confused with the main definition, this commit replaces those usages with the wording 'data buffer'.

Patch:
https://lists.oasis-open.org/archives/virtio-comment/202311/msg00632.html