tests: sampling: add initial HostStatsThread tests

This patch adds a unit test to HostStatsThread.

Change-Id: Ic4e5765364cf46e1ebfaac72aa5f75c80e83779e
Relates-To: https://bugzilla.redhat.com/1113948
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29566
Reviewed-by: Dan Kenigsberg <danken@redhat.com>