remote: fix systemd IP socket activation with virtproxyd

remote: fix systemd IP socket activation with virtproxyd

We recently forbid the use of --listen with socket activation:

  commit 3a6a725b8f575890ee6c151ad1f46ea0ceea1f3b
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Thu Aug 22 14:52:16 2019 +0100

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

In this change we forgot that virtproxyd doesn't have a --listen
parameter, and instead behaves as if it was always present. Thus
when systemd socket activation is present, we must disable this
built-in default

Reviewed-by: Erik Skultety <eskultet@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>