core: don't use hotplug when updating instance type

when updating vms connected to an instance type,
if using hotplug (for cpu) and it fails, the change is lost.

so always use "apply later" so the changes would anyway apply when the
vm is down.

Change-Id: I8f5b494a91ef3328fb2631da922c8a3cd582f67b
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1136372
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>