engine: Remove duplicate properties of VmTemplate

Remove fields interfaces and diskList with corresponding accessor
methods duplicitly declared on VmTemplate while being inherited from
VmBase with no additional logic except for overriding said accessor
methods to the same implementation.

Change-Id: I765da987d21ddf7153715322be9b9156b2a0ddca
Signed-off-by: Martin Betak <mbetak@redhat.com>