https://bugzilla.redhat.com/show_bug.cgi?id=1448149
If a domain has no numa nodes, that means we don't put any
memory-backend-file onto the qemu command line. That in turn
means we can't set access='shared'. Therefore, we should produce
an error instead of ignoring the setting silently.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>