CP-6167: Fix an issue reported by pylint
CP-6167: Fix an issue reported by pylint

The original code overwrite the 'add'/'remove' functions with
internal ones. This is considered as a bad practice as definitions
could be shadowed accidentally. Rename them to _add/_remove.

Signed-off-by: Zheng Li <fff69bf00b8ec5b6098a1768eaaaa75f40b8795d@eu.citrix.com>

djs55/sm,pritha-srivastava/sm,siddharthv/sm,xapi-project/sm,malcolmcrossley/sm,chandrikas/sm,geosharath/sm,martinjorge/sm,djs55/sm,geosharath/sm,xapi-project/sm,robertbreker/sm,djs55/sm,pritha-srivastava/sm,chandrikas/sm,BobBall/sm,chandrikas/sm,malcolmcrossley/sm,BobBall/sm,malcolmcrossley/sm,kostaslamda/sm,martinjorge/sm,xapi-project/sm,pritha-srivastava/sm,siddharthv/sm,kostaslamda/sm,martinjorge/sm,kostaslamda/sm,robertbreker/sm,siddharthv/sm,germanop/sm,BobBall/sm,robertbreker/sm,germanop/sm,germanop/sm,geosharath/sm