8. core: Change CreateAllSnapshotsFromVmCommand to persist all placeholders

This patch changes CreateAllSnapshotsFromVmCommand and its child
CreateSnapshotCommand to persist all async task placeholders
before executing the command on vdsm.

http://www.ovirt.org/Features/AsyncTaskManagerChanges_3.3
Change-Id: Ia0dda1086792f40a487488ac8a02b6b89dcfa9da
Signed-off-by: Ravi Nori <rnori@redhat.com>