qapi: Pass file name to QAPIGen constructor instead of methods

Not much of an improvement now, but the next commit will profit.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190301154051.23317-4-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
3 files changed