core: minor cleanups in MemoryImageRemover class

1. Define constant instead of using '6' explicitly in the code
2. Rename shouldRemoveMemorySnapshotVolumes method to
   isMemoryStateRemovable
3. Add log printing when the given memory state representation is not in
   the expected format (doesn't have 6 UUIDs separated by comma)
4. Rename local fields
5. Remove 'vm' and 'cachedPostZero' since they are not used by this
   class (so they are moved to the subclasses)
6. Add a boolean class field which indicates whether to start polling
   the created tasks or not, instead of indicating that by setting a
   parameter of the removeMemoryVolumes method

Change-Id: I14cb0bb13d0d103bbc170abcf0348a6909051f2c
Signed-off-by: Arik Hadas <ahadas@redhat.com>