vdsm: Load softdog if hardware watchdog is not available

On Ubuntu softdog is not loaded automatically. When there is not
hardware watchdog available, we have to load softdog to make wdmd and
sanlock work properly.

This patch detects /dev/watchdog during vdsmd startup and load softdog
if /dev/watchdog does not exist.

Change-Id: I17ca3dea7acd60b0ba5bb3634c06b656b41dde1d
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/17814
Reviewed-by: Dan Kenigsberg <danken@redhat.com>