restapi: Make adding (virtio-)console to virtual machines optional

This is rest part of a feature that allows users to control attaching
virtio console to vm/pools/templates.

Change-Id: I733e4c4a9c7d8f5dbe68b6b26a49502b8a40ec9d
Bug-Url: https://bugzilla.redhat.com/878459
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>