virtio_console: enable VQs early

virtio spec requires drivers to set DRIVER_OK before using VQs.
This is set automatically after probe returns, virtio console violated this
rule by adding inbufs, which causes the VQ to be used directly within
probe.

To fix, call virtio_device_ready before using VQs.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>