GitLab

Without this, we won't be able to do any flushes, so let's just require
it. Should be absent in very strange configurations.
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>