engine : snapshot creation task is not polled by engine

When the connectivity to storage domain is shutdown during
live storage migration flow, the async tasks take a long
time to clear after the connectivity wth storage domain
has been restorted. This results in the VM disks being
locked for a long time.

Change-Id: I78cb581b9c6ef350070e5ad53396daa069b0b4ea
Bug-Url: https://bugzilla.redhat.com/1078095
Signed-off-by: Ravi Nori <rnori@redhat.com>