This will create a respawn behaviour in case libvirt
process exits due to nonzero exit code, is terminated
by a signal, an operation times out or the configured
watchdog timeout is triggered.
see http://www.freedesktop.org/software/systemd/man/systemd.service.html
and https://bugzilla.redhat.com/show_bug.cgi?id=981974Signed-off-by: NEric Blake <eblake@redhat.com>