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

This patch makes it possible to control attaching virtio-console for
VMs/templates/VMs in pool.

The console is a managed device, therefore corresponding device type was
added. Also a command for querying DB for console devices was added.

Change-Id: I5d913b1fca9d064424aab58d4c34ec3e4b246373
Bug-Url: https://bugzilla.redhat.com/878459
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>