Change default resource partition to /machine

Change default resource partition to /machine

After discussions with systemd developers it was decided that
a better default policy for resource partitions is to have
3 default partitions at the top level

   /system   - system services
   /machine - virtual machines / containers
   /user    - user login session

This ensures that the default policy isolates guest from
user login sessions & system services, so a mis-behaving
guest can't consume 100% of CPU usage if other things are
contending for it.

Thus we change the default partition from /system to
/machine

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>