tests: fix wrong element checks in testAddDisk in machineslib.py
tests: fix wrong element checks in testAddDisk in machineslib.py

Commit 4209ce7 incorrectly introduced checks for add-disk-dialog element
id but this id is not present.

deryni/cockpit,deryni/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,cockpituous/cockpit,cockpituous/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpituous/cockpit,garrett/cockpit,garrett/cockpit,mvollmer/cockpit,deryni/cockpit,andreasn/cockpit,andreasn/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,andreasn/cockpit,deryni/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpituous/cockpit,martinpitt/cockpit,andreasn/cockpit,mvollmer/cockpit,andreasn/cockpit,andreasn/cockpit,andreasn/cockpit,moraleslazaro/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,deryni/cockpit,mvollmer/cockpit