engine: Fixed query to return task for volume

The query that returns the task associated with
volume had a missing paranthesis and hence the
volume id comparison was ORed instead of ANDed.

Change-Id: I55a0c3c472bc9a2edead4ed56dd455f61da1834a
Bug-Url: https://bugzilla.redhat.com/1061725
Signed-off-by: Sahina Bose <sabose@redhat.com>