vdsm-tool: handle missing sanlock pid file

Change-Id: I711d9b413415ca9372520e6bb25cf6d39ff571a3
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19112
Reviewed-by: Dan Kenigsberg <danken@redhat.com>