Remove Collector.instance

The virtual function Collector.instance can be replaced with general logic which
avoids the need to redefine some boiler-plate code for each new Collector.

Change-Id: Icb49d7397fadae141caad61033151d527ea9752c
Signed-off-by: Adam Litke <agl@us.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/13762