daemon: Fix LIBVIRT_DEBUG=1 default output

daemon: Fix LIBVIRT_DEBUG=1 default output

This commit changes the behavior of LIBVIRT_DEBUG=1 libvirtd:

$ git show 7022b09111d4322d21396a70d58320a9ad773962
commit 7022b09111d4322d21396a70d58320a9ad773962
Author: Daniel P. Berrange <berrange@redhat.com>
Date:   Thu Sep 27 13:13:09 2012 +0100

    Automatically enable systemd journal logging

    Probe to see if the systemd journal is accessible, and if
    so enable logging to the journal by default, rather than
    stderr (current default under systemd).

Previously  'LIBVIRT_DEBUG=1 /usr/sbin/libvirtd' would show all debug
output to stderr, now it send debug output to the journal.

Only use the journal by default if running in daemon mode, or
if stdin is _not_ a tty. This should make libvirtd launched from
systemd use the journal, but preserve the old behavior in most
situations.