[Libvir] Adding suppport for daemon restarts with stateful drivers

[Daniel P. Berrange]

The libvirt daemon has the ability to reload itself by sending it SIGHUP. 
For the QEMU & network drivers this makes it reload the config files for
VMs and re-init the iptables rules. It would be desirable though to allow
the daemon to perform a full restart. Principally this is for RPM upgrades
where you want toensure the daemon is running the new code.

The tricky thing is figuring out how to handle driver state. Looking at the
QEMU, network, storage and LXC drivers, there is not actually all that  much
state to deal with. It basically comes down to:

 - PID of child processes (eg QEMU, dnsmasq, container)
 - FDs for STDIN/OUT/ERR of the child processes
 - A possible logfile FD
 - Flag to indicate whether some objects are active or not

That is more or less it. Anything else is kept in the config files and can
be reloaded at will.

So I was thinking about whether we could provide a simple protocol to allow
each stateful driver to save its state into some location, the daemon could
just 'exec()' itself again, and upon startup the drivers reload their active
state. Since the daemon just exec()'s itself it would still own the child
processes & still have all the neccessary FD's open. 

Dan.
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