machines: testAddDisk: add wait_present for element before accessing it
machines: testAddDisk: add wait_present for element before accessing it

It rarely failed on trying to access this element, but since I added the
check is passes.

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