ch: Load ch.conf from SYSCONFDIR

ch: Load ch.conf from SYSCONFDIR

Config files for system instances of our drivers (e.g.
"ch:///system", "qemu:///system", etc.) live under /etc/libvirt.
But for some reason, the CH driver was trying to load the config
file from /var/lib/libvirt/ch/ even though the file is installed
under /etc/libvirt per the following line from src/meson.build:

  install_data(virt_conf_files, install_dir: confdir)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>