virDomainCoreDumpWithFormat: Mention enum for @dumpformat

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>