conf: Reject invalid device's <seclabel relabel='yes'/> with no <label/>

conf: Reject invalid device's <seclabel relabel='yes'/> with no <label/>

We allow (some) domain devices to have a different <seclabel/>
than the top level domain one (this is mostly to allow access to
a resource for multiple domains). Now, we do couple of sanity
checks for such <seclabel/>, e.g. when the <label/> is specified,
but '@relabel' is set to no. But what we are missing is the
opposite: when '@relabel' is set, but no <label/> was provided.

Our schema already denies such combination. Make our parser
behave the same.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2160356
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>