core: add managed virtio-serial devices on upgrade

The upgrade scripts do the following things:
- Change every existing virtio-serial device to be managed
- Then is adds managed virtio-serial device to VMs that don't have it

In addition, we might get reports of unmanaged virtio-serial devices
from VMs that were running at the time of the upgrade, so in that case
we'll not add new device, but use the managed device instead.

Change-Id: I5b99dd3fa0d43307788a685c0115b32691757454
Bug-Url: https://bugzilla.redhat.com/1028387
Signed-off-by: Arik Hadas <ahadas@redhat.com>