Removal of needless base class StatsThread

From this base class inherit only class HostStatsThread, therefore can
be content of class StatsThread moved to class HostStatsThread without
changing of functionality and with higher clarity.

Change-Id: Ica913d6e97d37c7729b77574373247add2125338
Signed-off-by: Petr Sebek <psebek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20745
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>