machines: Check for missing XML domain element
machines: Check for missing XML domain element

Make sure that the domain XML actually contains a domain element, to
avoid a crash on undefined `domainElem()`. This does not really change
behaviour, just improves the error message a bit and quiesces coverity:

    property_access: Accessing a property of null-like value "domainElem".

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