core: fix vm images list initialization

Move the initialization of the images list of VM to be in VmBase instead
of in VM so when we instantiate VM with an instance of VmStatic which
includes images list, those images will be used and will not be
overridden.

Change-Id: If7d713935c2449340ae01bd705c2edf907fbd1cd
Signed-off-by: Arik Hadas <ahadas@redhat.com>