core: remove memory volumes without tasks on snapshot creation failure

On the end-action phase of snapshot creation command, we check if the VM
is runningto see whether we should take live snapshot or not. If the VM
is not running and we created images that will be used to store the
memory, we try to remove those images. There was a problem that the
remove image operations created tasks that were not polled (they are not
polled automatically since we're in the end-action phase, and we didn't
explicitly asked to poll them).

Since we have nothing to do in case of failure (if the file deletion
didn't work, it most probably won't work on retry), we now don't create
tasks on those delete image operations.

New command RemoveMemoryVolumesCommand is added to do so, to achieve
better separation of concerns, which will also make it possible to reuse
it on other cases.

Change-Id: I3830992f46b2c3fbea47e69f21a6711718408e95
Bug-Url: https://bugzilla.redhat.com/1011474
Signed-off-by: Arik Hadas <ahadas@redhat.com>