remote: forbid the --listen arg when systemd socket activation

remote: forbid the --listen arg when systemd socket activation

When using systemd socket activation the --listen arg has no
effect. This is confusing to users upgrading from previous versions of
libvirt as their config is silently ignored. Turn use of --listen into a
fatal error when sockets are passed from systemd.

This helps the admin discover the change in behaviour and thus decide
whether to stick with socket activation or revert to previous behaviour.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>