core: improve post zero check

This patch improves the check whether to use post zero or not when
removing memory state images using MemoryImageRemover class: instead of
filtering the list of disks to get all the disks which are marked with
"wipe after delete" and then check if the list is not empty, we check
whether one of the disks in the list is marked with "wipe after delete".

This change has several benefits:
1. we don't have to go over all the items in the list everytime (only in
the worst case, if no such disk exists in the list)
2. we don't need to create new list
3. the code is shorter and is more intuitive

Change-Id: Id78a5b74748f5c174fd0fba70c95c40b66ff247f
Signed-off-by: Arik Hadas <ahadas@redhat.com>