link monitor: do not leave stray `ip monitor` behind

ipwrapper.Monitor.start() spawns an `ip monitor link` process that may
run forever. If Vdsm crashes, we should take care to kill it, or it
would leak.

Due to this patch, ipwrapper.Monitor.start() should be called from an
ever-lasting thread.

Change-Id: Icb1f8998a3f71046c28bf902c851bf900b1e6f7f
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22667
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>