engine : VMs pool disappeared after creating a Sub Template version

VMs pool disappeared after creating a Sub Template version
with exception constructor not found

Change-Id: I7a4833182c3e9302308520a661e1318e366cb04d
Bug-Url: https://bugzilla.redhat.com/1138419
Signed-off-by: Ravi Nori <rnori@redhat.com>