engine: Change default 64-bit memory limit on upgrade

Changes of memory limit for 64-bit VMs by
14dcb41a97be002833d361adfdf7ac199743d671 are now reflected on upgrade, not only
on clean install.

Change-Id: I8dbe4747ce6e8b17c4bfff96cb8a81599632b7da
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=954021
Signed-off-by: Martin Betak <mbetak@redhat.com>