[Libvir] Restarting of libvirt_qemud daemon

[Daniel P. Berrange]

Thinking about later RPM upgrades I think we need to think about whether it
will be possible to restart the libvirt_qemud while guests & networks are
running. There's a couple of issues:

 - We do waitpid() to cleanup qemu & dnsmasq processes when we stop domains
   & networks, or to detect when they crash. For the former, we could may
   they daemons to avoid waitpid() cleanup, but we'd still need waitpid to
   be able to detect shutdowns. There is also the issue of enumerating
   running instances.

 - We always try to re-create a bridge device at startup, even if it already
   exists. Likewise we always try to add the IPtables rules & start dnsmasq.
   We can easily detect if the bridge already exists. I think we can probably
   double check iptables rulles too., The tricky one is figuring out whether
   a dnsmasq instance is still running.

Dealing with theses not only helps planned restarts, but will also make it
possible start up the daemon again after a crash without having to kill off
all guests & networks manually. So I think it is worth investigating what
we can do to enable restarts. It might be worth waiting until we sort out
whether we'll merge libvirt_qemud with the generic libvirtd remote daemon
though so we don't have to do the work twice over.

Dan.
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