core: fix findbugs errors

There were couple of errors related to the way we compute if post zero
is needed when removing memory image from export domain. This patch
changes the way we do it to be more standard.

Change-Id: Ic66ba287b6048615b71cd0b2d876e7d75bcc4655
Signed-off-by: Arik Hadas <ahadas@redhat.com>