core: improve logging on memory volumes removal failure

Change the logging that is printed when failing to remove memory volumes
to provide more information.

This patch also contains some refactoring which should make the
'endVmCommand' method in create-all-snapshots command more simple and
readable.

Change-Id: I67897b5cd07033aff624242cdaa67d517eb14bf9
Signed-off-by: Arik Hadas <ahadas@redhat.com>