Leave all child processes running when stopping systemd service

Leave all child processes running when stopping systemd service

Currently the libvirt.service unit file for systemd does not
specify any kill mode. So systemd kills off every process
inside its cgroup. ie all dnsmasq processes, all virtual
machines. This obviously not what we want. Set KillMode=process
so that it only kills the top level process of libvirtd

* daemon/libvirtd.service.in: Add KillMode=process

Reported-By: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>