virtio: remove unused next argument from virtqueue_split_read_next_desc()

virtio: remove unused next argument from virtqueue_split_read_next_desc()

The 'next' was converted from a local variable to an output parameter
in commit:
  412e0e81b174 ("virtio: handle virtqueue_read_next_desc() errors")

But all the actual uses of the 'i/next' as an output were removed a few
months prior in commit:
  aa570d6fb6bd ("virtio: combine the read of a descriptor")

Remove the unused argument to simplify the code.

Also, adding a comment to the function to describe what it is actually
doing, as it is not obvious that the 'desc' is both an input and an
output argument.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Message-Id: <20230927140016.2317404-3-i.maximets@ovn.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>