formatter: clean up dump_format()

Create a common dump_format_va() function, and make all the different
variants call it.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #2407 from ceph/wip-formatter-va

formatter: clean up dump_format()

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>