engine: Lazily initialize VM interfaces list

In order to deallocate vNICs from external networks, the interfaces list
for the VM is necessary. Therefore, the interfaces list will be
initialized lazily.

Change-Id: Ib2c08ea82134b398344dfed41e5000de2fe98a74
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>