core: OvfReader - set IsManaged to true for managed devices

When reading ovf data, managed devices were added to the managed devices
map - but the isManaged flag wasn't set to true, causing that possibly
wrong data would be persisted/used.

Related to Bug-Url: https://bugzilla.redhat.com/882644

Change-Id: Ifa51f5aa6655b5b9ae6fd1a8c5e0c206fa9630e6
Signed-off-by: Liron Aravot <laravot@redhat.com>