kvm tools: Introduce --enable-virtio-console option

kvm tools: Introduce --enable-virtio-console option

This option enables virtio console and disables serial console.
At this time, the virtio and serial console can not work simultaneously.
Becasue we can not put both console's output to user at the same time.
However, once we redirect the virtio console to a pipe or socket. We can
use both console simultaneously.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>