core: set boot order index 0 for unmanaged nic

This patch fix NPE which was thrown when computing boot order for a VM
that has unmanaged network interface. The boot order index for unmanaged
network interfaces will now be 0.

Change-Id: If299f5c10dbfd15cf5ebe01f6d7911a7f92eb56f
Bug-Url: https://bugzilla.redhat.com/1155667
Signed-off-by: Arik Hadas <ahadas@redhat.com>