core: simplified memory states removal code

This patch simplifies the
MemoryImageRemover#removeMemoryVolumes(String, boolean) method:
1. split the long method into several methods
2. reorganize the tasks polling related code

Change-Id: Ib0b5b9d77458db7c2a7e97fe0d36fd2b8679d20d
Signed-off-by: Arik Hadas <ahadas@redhat.com>