core: fix regression related to update pool

Because of the use of do-while to add the missing VMs in pool after
updating the pool, we were adding a VM also in case no new VMs where
requested (because of the 'do' part).

The solution is to replace the do-while with for loop.

Change-Id: I027d687ecca87dd5eb784e99cd14b115ad55903f
Signed-off-by: Arik Hadas <ahadas@redhat.com>