virtio-sound: implement audio capture (RX)

virtio-sound: implement audio capture (RX)

To perform audio capture we duplicate the TX logic of the previous
commit with the following difference: we receive data from the QEMU
audio backend and write it in the virt queue IO buffers the guest sends
to QEMU. When they are full (i.e. they have `period_bytes` amount of
data) or when recording stops in QEMU's audio backend, the buffer is
returned to the guest by notifying it.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <e56a17741a24ccadfbbea19d3c60c9406b795b23.1698062525.git.manos.pitsidianakis@linaro.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>