domainMonitor: Improve logging

We have logs in domain monitor thread, and we can see when a monitor
starts or stops, but we don't have any info which thread triggered the
operation.  This patch adds info level log messages when starting and
stopping domain monitoring, revealing the caller thread.

Change-Id: I20feb40b097fa65ebebf851e33c051fc184ff029
Relates-To: https://bugzilla.redhat.com/1032925
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21904
Reviewed-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Ohad Basan <obasan@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>