tools/hotplug: xendomains.service conflicts with libvirt

tools/hotplug: xendomains.service conflicts with libvirt

xendomains will manage guests behind libvirts back:
- libvirt starts a guest
- that guest can be "managed" by libvirt and xl at the same time
- when xendomains runs on shutdown it will save the guest using xl
  libvirt does not know about this
- when xendomains runs on boot it will restore the saved guest using xl
  libvirt does not know about this, it will just fail to manage the
  restored guest

To prevent xendomains from interfering with libvirt add a Conflicts= to
xendomains.service. It will cause libvirt to be stopped if xendomains is
started manually with 'systemctl start'.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>