Move machineName generation from virsystemd into domain_conf

Move machineName generation from virsystemd into domain_conf

It is more related to a domain as we might use it even when there is
no systemd and it does not use any dbus/systemd functions.  In order
not to use code from conf/ in util/ pass machineName in cgroups code
as a parameter.  That also fixes a leak of machineName in the lxc
driver and cleans up and de-duplicates some code.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>