CP-6167: Fix a bug reported by pylint
CP-6167: Fix a bug reported by pylint

from_tapdisk is a classmethod, it doesn't have access to 'self'.

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

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