Don't ignore all dbus connection errors

Don't ignore all dbus connection errors

Previous commit

  commit 7ada155cdf2bbfac16ce08f64abb455a940e2cf7
  Author: Gao feng <gaofeng@cn.fujitsu.com>
  Date:   Wed Sep 11 11:15:02 2013 +0800

    DBus: introduce virDBusIsServiceEnabled

Made the cgroups code fallback to non-systemd based setup
when dbus is not running. It was too big a hammer though,
as it did not check what error code was received when the
dbus connection failed. Thus it silently ignored serious
errors from dbus such as "too many client connections",
which should always be treated as fatal.

We only want to ignore errors if the dbus unix socket does
not exist, or if nothing is listening on it.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>