task: prevent tasks with the same id to be queued

This patch prevents a task to be queued if its id is already used
by another task. Previously the task already present in the queue
would have been lost.

Change-Id: I0b9980de08d10203a75071dae9b16c9b850a91f5
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26808
Reviewed-by: Dan Kenigsberg <danken@redhat.com>