virtio: console: Wake up outvq on host notifications

The outvq needs to be woken up on host notifications so that buffers
consumed by the host can be reclaimed, outvq freed, and application
writes may proceed again.

The need for this is now finally noticed when I have qemu patches ready
to use nonblocking IO and flow control.

CC: Hans de Goede <hdegoede@redhat.com>
CC: stable@kernel.org
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Hans de Goede <hdegoede@redhat.com>