virDomainCoreDumpWithFormat: Mention enum for @dumpformat
    
    So the API takes @dumpformat argument. This is what makes it special
    when compared to virDomainCoreDump. The argument is there so that
    users can choose the format of resulting core dump file. And to ease
    them the choosing process we even have an enum with supported values
    across all the hypervisors. But we don't mention the enum in  the
    function description anywhere. Fix it!
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>