core: Make commited_disk_size view column show total virtual size

In the storage views the commited_disk_size column is supposed to show the
total amount of the disks virtual sizes (commited size) in practice this
column shows the commited size minus the actual size of the disk, this
was fixed to show the commited size alone as the actual size is shown
already in used_disk_size and is not needed

Change-Id: Ic16b249779add146705a4e104dd93836923fbbd4
Bug-Url: https://bugzilla.redhat.com/1025294
Signed-off-by: Tal Nisan <tnisan@redhat.com>