engine: pooled VMs was not sysprepped

The problem was that the is_initialized has been ignored for
pooled VMs. Now changed that the pooled and not pooled VMs
are sysprepped under the same conditions (windows VMs with
is_initialized flag set to false).

Change-Id: I65c69514e3edd02ff44685f58bccd7f0674ef8dd
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>