9pfs/virtio: implement response_buffer_size callback Add and implement the response_buffer_size callback for the virtio transport. Returns the actual current virtio response buffer size for the supplied PDU, which will be used as safety guard for limiting the response size when generating a 9p response. Link: https://lore.kernel.org/qemu-devel/5bbed2768f7a0da8fa2be183e75928c5d1ef691d.1781287774.git.qemu_oss@crudebyte.com Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>