qapi: Improve reporting of invalid name errors

qapi: Improve reporting of invalid name errors

Split check_name() into check_name_is_str() and check_name_str(), keep
check_name() as a wrapper.

Move add_name()'s call into its caller check_exprs(), and inline.

This permits delaying check_name_str() there, so its error message
gains an "in definition" line.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-10-armbru@redhat.com>