Change default cgroup layout for QEMU/LXC and honour XML config

Change default cgroup layout for QEMU/LXC and honour XML config

Historically QEMU/LXC guests have been placed in a cgroup layout
that is

   $LOCATION-OF-LIBVIRTD/libvirt/{qemu,lxc}/$VMNAME

This is bad for a number of reasons

 - The cgroup hierarchy gets very deep which seriously
   impacts kernel performance due to cgroups scalability
   limitations.

 - It is hard to setup cgroup policies which apply across
   services and virtual machines, since all VMs are underneath
   the libvirtd service.

To address this the default cgroup location is changed to
be

    /system/$VMNAME.{lxc,qemu}.libvirt

This puts virtual machines at the same level in the hierarchy
as system services, allowing consistent policy to be setup
across all of them.

This also honours the new resource partition location from the
XML configuration, for example

  <resource>
    <partition>/virtualmachines/production</partitions>
  </resource>

will result in the VM being placed at

    /virtualmachines/production/$VMNAME.{lxc,qemu}.libvirt

NB, with the exception of the default, /system, path which
is intended to always exist, libvirt will not attempt to
auto-create the partitions in the XML. It is the responsibility
of the admin/app to configure the partitions. Later libvirt
APIs will provide a way todo this.

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