RFC: qemu: use uuid instead of name for misc filenames

Hi, everyone. I'm working on supporting domain renaming when it has snapshots which is not supported now. And it strikes me things will be much simplier to manage on renaming if we use uuid in filenames instead of domain names. 1. Renaming will only involve saving updated config. The saving is atomic thanx to tmp file and rename(2) approach. In constast current renaming on error paths can leave config with old or new name. Thus on libvirt restart extra VM will appear. And we don't need to rename autostart links, snapshot directories etc. 2. Renaming will be possible for running domains with no efforts. We only need to pass uuid instead of name in '-name guest=...' command line. 3. Mgmt can stop using autogenerated names for domains. I guess openstack for example uses names like instance-000002ff because we have many limitations on domain renaming. And if these limitations are removed then openstack can just use user supplied names for domains. 4. No issues with long domain names and filename length limit If the above conversion makes sense I guess the good time to apply it is on domain start (and rename to support renaming with snapshots). I guess we can also have tool (some virsh command) for developers to generate symlinks so one can access logs, configs etc by name instead of uuid. Nikolay