virconf: Fix config file path construction

virconf: Fix config file path construction

Since commit c4bdff19, the path to the configuration file has been constructed
in the following manner:
 - if no config filename was passed to virConfLoadConfigPath, libvirt.conf was
 used as default
 - otherwise the filename was concatenated with
 "<config_dir>/libvirt/libvirt%s%s.conf" which in admin case resulted in
 "libvirt-libvirt-admin.conf.conf". Obviously, this non-existent config led to
 ignoring  all user settings in libvirt-admin.conf. This patch requires the
 config filename to be always provided as an argument with the concatenation
 being simplified.

 Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1357364

Signed-off-by: Erik Skultety <eskultet@redhat.com>