core: update the committed memory properly

The computation of the committed memory was wrong because it included
VMs which are down as well. The problem was that we were iterating the
wrong collections of VMs, we should iterate over the VMs which we got
back from VDSM and take only them into consideration.

Change-Id: I04be4ca80456ffbf47af2f883a33672166c25cb6
Bug-Url: https://bugzilla.redhat.com/1078553
Signed-off-by: Arik Hadas <ahadas@redhat.com>