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

Undefined variable "blockdevice" was referenced in the code. It
should be "self.blockdevice".

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

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