virtio_console: Stop doing DMA on the stack

virtio_console uses a small DMA buffer for control requests.  Move
that buffer into heap memory.

Doing virtio DMA on the stack is normally okay on non-DMA-API virtio
systems (which is currently most of them), but it breaks completely
if the stack is virtually mapped.

Tested by typing both directions using picocom aimed at /dev/hvc0.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>