[libvirt] [RFC] move host uuid out of libvirtd.conf (effectively)

Hi, all. I hope RFC flow from me are not hitting some rate limiting. Integration tests in our compary undercover that host uuid provided thru dmi is not as unique as it should be. This is probably a manufacturer mistake but still the case. Here libvirt's config field host uuid comes to the rescue. It is a good option in case of tweaking a few machines. In case of full-fledged mgmt it is probably enough too. But in case your managment is managment of packages thru you package manager it is not that convinent. The problem is that if one generates uuid on package install then package update will not be easy. One probably wants for updates to go smoothly in case user does not tweak anything but now it is not possible as changing host uuid is visible as user intervention by package managment. I'm talking now from my rpm based managment experience but I imagine this could be no different in other packaging systems too. This is probably could be overcomed thru some kind of reserse patching before update and patching again after if package system is wise enougth to check file hashes and not timestamps for example. But then this is should be done in this and every packaging system. So my proposition is simple - make it possible to take host uuid from distinct file. I think keeping uuid could be the sole purpuse of it. Then we can generate uuid on package installation and later updates need no any extra actions at all. Another benefit of keeping uuid in distinct place is that one can just copy daemon configuration to a different host not being aware it needs to be changed.