make xendomains service save/migrate vms under /etc/xen/auto only

make xendomains service save/migrate vms under /etc/xen/auto only

Currently, xendomains service will save or migrate all the running domains in
the host on stop.

Change XENDOMAINS_AUTO_ONLY=true. Then on service stop, it will only
save or migrate all VMs under XENDOMAINS_AUTO (/etc/xen/auto), but always
shutdown all the running VMs.

It's a way to cleanly shutdown all the running VMs on system reboot. If you
want to save/migrate a VM, create a link to /etc/xen/auto.

Signed-off-by: Zhigang Wang <zhigang.x.wang@oracle.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>