virt: migration: use running() for downtime thread

Now that migration.DowntimeThread() conforms to the
interface required by utils.running(), we can make
use of this context manager and make the code a little
nicer and shorter.

Change-Id: I5418b610fc1746751b717ad7d3c746a548559566
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33032
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>