core: Fix task polling when exporting vm command with memory snapshots

When exporting vm with memory snapshots that tasks are not polled,
causing to the command end method to be called possibly
too early and to the tasks to never be cleared.

Change-Id: Ief3b3a361fd0ff175f333c7ac2935ce6993ae4b2
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1029324
Signed-off-by: Liron Aravot <laravot@redhat.com>