virtio-console: Automatically use virtio-serial-bus for the older -virtioconsole...

virtio-console: Automatically use virtio-serial-bus for the older -virtioconsole invocation

These hunks got dropped off mysteriously during the rebasing of my
virtio-serial series. Thanks go to Markus for noticing it.

Without these fixes, -virtioconsole doesn't actually have any effect.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>