kvm tools: Make --virtio-9p option shorter

This patch changes --virtio-9p to --9p because:

1. It's shorter
2. No need to add virtio prior to 9p since we use virtio by default

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