qapi: Inline check_name() into check_union()

check_name() consists of check_name_is_str() and check_name_str().
check_union() relies on the latter to catch optional discriminators.
The next commit will replace that by a more straightforward check.
Inlining check_name() into check_union() now should make that easier
to review.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-16-armbru@redhat.com>