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>

tests/virsystemdtest.c

@@ -413,8 +413,8 @@ testMachineName(const void *opaque)
     int ret = -1;
     char *actual = NULL;
 
-    if (!(actual = virSystemdMakeMachineName("qemu", data->id,
-                                             data->name, true)))
+    if (!(actual = virDomainGenerateMachineName("qemu", data->id,
+                                                data->name, true)))
         goto cleanup;
 
     if (STRNEQ(actual, data->expected)) {