sampling: make HostStatsThread a daemon thread

It can cause VDSM to hang when being improperly shut down.

Change-Id: Ia20ded74241bac631fd177f4d7f0b0cda2c4fcbc
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1022036
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Signed-off-by: Yaniv Bronhaim  <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22237
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>