vdsm: No need to 'import threading'

This fixes the build after "vdsm: making PM 'on' and 'off' sync".

Change-Id: Id306f8c897efd6985aaf260d168376c9f1f031e3
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33496
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>