virt: migration: avoid implicit thread start

migration.DowntimeThread used to implicitely
start() itself at the end of its __init__().

This is bad practice, so this patch removes
the implicit start and make the method be called
when needed.

Change-Id: I6859be9ce73c618f824e9f5cc0da8fcfe550ac2f
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33031
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>