virtio: console: Prepare for writing to userspace buffers

When ports get advertised as char devices, the buffers will come from
userspace. Equip the fill_readbuf function with the ability to write
to userspace buffers.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>