libvirtstats plugin: Add the `HostnameFormat' option.

libvirtstats plugin: Add the `HostnameFormat' option.

This is something I missed out of the previous patch: I want a way to log
guests using their UUID instead of their name (more useful for us because of
the migration and other issues outlined before).

This adds:

   <Plugin libvirtstats>
     HostnameFormat "uuid"
   </Plugin>

to let us do that. You can do other things as well such as:

   HostnameFormat "name uuid"

to get "foo:1234-1234-1234-1234" as the hostname.

Rich.